The distribution of the seats in the Volkskammer was predetermined by the National Front, an "alliance" of political parties that was completely controlled by the SED, and the only "right" people had to vote was either to agree to the list (which 99%+ of people did) or disagree. The composition of the Volkskammer was essentially static through the whole existence of the DDR.
What was the purpose of the minor parties when it would be immediately clear to anyone that the SED was in charge? Why would someone join a minor party? Was it a good move, career-wise, to be in a minor party, or was it politically risky? Especially parties like the LDPD which combined with the FDP after re-unification, as a liberal and anti-communist party, surely some of the members were suspect?
The bloc parties - officially the "National Front of the German Democratic Republic" for most of its existence - did serve a number of roles in East German society and politics, though much of their utility served the Socialist Unity Party (SED), rather than the various parties' members. I'm going to begin this answer by providing some background on the National Front that may (or may not) be unnecessary for you, but might help those totally unfamiliar with the way party politics worked in the GDR.
The parties of the National Front were, as you say, effectively controlled by the SED. There was very little ambiguity about this. Eventually, it was even written into the GDR's constitution. Article 1 of the 1968 Constitution stated that "[t]he German Democratic Republic ... is the political organization of the working population in town and country, who are jointly realizing socialism under the leadership of the working class and its Marxist-Leninist party." The identity of this Marxist-Leninist Party was not subject to doubt or debate. This system was and is not unique to East Germany. Other Soviet satellite states did the same - see, e.g., the Czechoslovakian National Front and the political structure of the Polish People's Republic - and North Korea has a similar arrangement today.
All of the bloc parties were the product of the immediate post-war years, during which there was a certain degree of political flexibility and freedom in Soviet-occupied Germany which allowed for the establishment of multiple parties, not all of whom were communist, nor even socialist. On 10 June 1945, the Soviet Military Administration in Germany declared that "permission is to be given for the formation and activities of all anti-fascist parties having as their aim the final eradication of the remnants of fascism, the consolidation of the foundations of democracy and civil liberties in Germany, and the development of the involvement and participation of the broad masses of the population in this direction." Two of the future bloc parties - the Liberal Democratic Party of Germany (LDPD) and the Christian Democratic Union (CDU) were both founded independently shortly after the war, representing political platforms comparable to those of liberals in western Germany. The 5 July 1945 founding program of the LDPD, for example, stated that "[t]he retention of a unified German national economy, of private property, and of the free market is a prerequisite for initiative and for successful economic activity. Subordination of businesses to public control is justified only if the enterprises concerned are suitable and ready for this, and if this is required in the overwhelming interest of the common good."
Of course, this relatively permissive approach to party politics was not long to last; by the time the German Democratic Republic was created out of the Soviet Occupation Zone in 1949 (and the National Front was created shortly thereafter), the Communists (with the patronage of the the Soviet Military Administration) had effectively neutered partisan political opposition at the leadership level.
The Liberal Democrats, Christian Democrats, Social Democrats, and Communists had joined together in a "United Front of the Anti-Fascist Democratic Parties" in July 1945, committing themselves to a loose collection of unobjectionable ideas - reconstruction of the economy, anti-fascism, freedom of conscience, international peace, etc. This was not yet the National Front, but it did establish a formal basis for inter-party cooperation that the Communists would later use to their decisive advantage. In 1946, the Social Democrats were pressed into a union with the Communists to form the SED. In 1948, two new political parties - the National Democratic Party of Germany (NDPD) and the Democratic Farmers' Party of Germany (DBD) - were founded more-or-less as "shell" organizations controlled by the SED. The Communists also consolidated their power within the National Front by including a variety of non-party organizations, such as the Free German Trade Union Federation (FDGB) and the Free German Youth (FDJ), all of which were controlled by the SED and/or entirely dependent on SED patronage for their power and survival. In 1950, the parties were forced through the National Front to assent to a "unity list" of candidates for the Volkskammer. This pattern would hold until the collapse of SED hegemony in autumn 1989.
As Walter Ulbricht had allegedly described it in 1945, the Communists believed that the system "must look democratic, but we must control everything." The National Front was an important part of their strategy of control. It served their purposes in several ways. First, the Front neutralized substantial political opposition by pushing it in directions that were unthreatening to the SED government. The NDPD and DBD are particularly stark examples of this, as they were created deliberately in order to attract the support of former Nazis and rural agricultural workers, respectively, drawing those troublesome constituencies away from alternative, oppositional political engagement.
Second, the Front allowed the SED to portray the East German system abroad as being simultaneously pluralistic and united. Pluralistic because it tolerated multiple parties that represented ostensibly different viewpoints, but united because all of the parties "voluntarily" accepted the leadership of the SED and the importance of establishing and defending "Socialism on German Soil." A 1974 article in the GDR journal Deutsche Außenpolitik described it thusly:
In the GDR the existing party alliance under the SED's leadership is characterized by all the parties' unreserved agreement in building the advanced socialist society. Two "key questions" which one of the parties puts to all its members as a basic requirement are characteristic for the attitude of all four non-communist parties - CDU, LPDP, NDPD, DBD - to the working class and its Marxist-Leninist vanguard and to world socialism. One asks for full recognition of the claim to leadership of the working class and the SED, and the other for a decisive declaration of support for the Soviet Union as the leading power in the socialist world system. ... The parties linked in friendship to the SED have their firm place in socialist society. They give important assistance for the whole of society when they contribute within their own circles to the formation of socialist state and property consciousness, to the further development of socialist relations of production, to the firm anchoring of the GDR in the socialist world system.
"[T]he political system," explained a press handbook produced by the GDR's Foreign Press Agency in 1986, "is characterized by trusting and comradely co-operation between the parties and mass organizations. ... Nowadays, it is the GDR's socialist popular movement which unites the political parties, mass organizations and individual citizens."