From the Soviet perspective, the Eastern Bloc was immensely important. It provided a sense of security in several ways: in a literal sense, it was a buffer between the West and the Soviet Union, most specifically West Germany which the Soviet Union always feared might again revamp its military and attack Russia. It also provided the Soviets with the sense that it had made gains in the world and that it held on to those gains. So long as the Bloc remained intact, the Soviets could rightly claim that it had ideological, political, and military allies in Europe, even if the Soviet Union had to intervene from time to time in their internal affairs (and in fact through the manipulation of the secret police this intervention went well beyond what the Soviets would have liked to admit). Soviet morale largely depended on the maintenance of the Warsaw Pact alliance, and it is no surprise that the fall of the Eastern Bloc helped to precipitate the collapse of the Soviet Union itself.

What you might really like to know is how much the Eastern Bloc actually contributed to Soviet interests, as opposed to satisfying the psychological needs I just described. Economically, the Bloc obviously helped the Soviet Union maintain a market mostly closed off from the global market--as is typical of an imperial power, it used its sphere as both a market for its own goods and as the source of raw materials it could not itself produce. To this end, it contributed greatly to the decline in the economic power of its satellites, most notably Czechoslovakia which had been one of the richest nations on earth prior to the Cold War and whose economists concluded in the 1960s that Czechoslovakia could still make gains with a controlled economy so long as it reopened itself to the world market (this venture, which the Soviet Union initially supported to a degree, ended with all other Czechoslovak reforms after the Warsaw Pact invasion of 1968). From a military standpoint the Eastern Bloc also served the Soviet Union's real interests, as it provided it with divisions that would resist an attack on the Soviet Union and Western points to deploy its short and medium range missiles--i.e., the advantage was not all psychological, even in a world now dominated by those with nuclear weapons.

The Eastern Bloc also presented the Soviet Union with problems that simply did not exist for its rival the United States, however. When one of its allies Hungary threatened to leave the Warsaw Pact in 1956, it opened up the very real possibility that the Soviet Union would soon (in 5 years? 10 years? certainly at some point) have NATO forces stationed on its border. Moreover, the collapse of Marxist-Leninism in any of its allies, whether it be Hungary in 1956 or Czechoslovakia in 1968, the latter of which never threatened to leave the Warsaw Pact, would likely have led to discontent and anti-Leninist activity throughout the Bloc. Leaders in Moscow therefore had to invade its own allies at times, straining not only relations within the Eastern Bloc but also relations between the Soviet Union and those nations around the world whose hearts and minds it hoped to capture. Compare this to the French rebellion within NATO during the same time period: the United States obviously didn't like what happened in France, but its national interests remained intact in Western Europe. It was not as if France might soon ally with the Warsaw Pact, nor would Warsaw Pact forces soon sit on the other side of the American border. And so Western Europe provided all the benefits that Eastern Europe provided to the Soviet Union, but without the glaring drawbacks described here.

TL;DR: The Eastern Bloc was psychologically, economically, and military important to the Soviet Union, but it also represented a major liability in that it was a Cold War construct that required constant maintenance, sometimes at the expense of Soviet prestige.