The Soviets gained the Porkkala Naval Base in the 1944 peace with Finland to end the Continuation War. They had originally sought the base as far back as 1938-1939, prior to the onset of the Winter War, because to the Soviets stationing their navy at Porkkala protected Leningrad (Jakobson's "Finland Survived" and "The Diplomacy of the Winter War" are two good books that provide the Finnish perspective, however Jakobson himself was a Finnish diplomat, so one must be cautious of bias; Trotter's "A Frozen Hell" gives a good summary of the military argument/battles that occurred during the Winter War, to include the proposals made by the Soviets- rejected by Finland- to get the Porkkala Naval Base).

The return of Porkkala was done as a good will gesture of sorts by Khrushchev once he took power following the death of Stalin. Essentially, Khrushchev's policy was to return foreign bases in countries that were not seen as a external threat, and were not members of the Warsaw Pact. Finland was just required to agree to extend the validity of the 1948 mutual defense treaty by another twenty years (the initial treaty was for a term of ten years; the treaty was extended multiple times after 1956 as well). This also included Soviet bases in Dairen and Port Arthur (China) and Jawalharlal Nehru (India). In the big picture, Khrushchev's plan was to try and pressure the United States to give up its overseas bases by claiming that the Soviet Union had done the same (ignoring the Warsaw Pact countries of course).

Finally, the nature of warfare and technology had changed so much that having a base on Porkkala was unnecessary. The Soviets possessed nuclear weapons, and they had weapons that could cover the approach to Leningrad from within their own borders, and didn't need a base right along the Baltic Sea to accomplish their security needs. Finland was the victor in all of this, as the removal of Soviet troops from their territory lended a lot of credit to their claims (and strong desire) for neutrality, both during the Cold War and even in the period since then.

* Details in the final two paragraphs were sourced from the third chapter of Max Jakobson's " Finland in the New Europe" which is an excellent starting point for understanding the post WWII aims and foreign policy of Finland on the international stage.