Because people seem to think Stalin was a tyrant acting as an autocrat and causing millions upon millions of deaths:
Thousands of decisions were taken at Politburo level that directly affected the real lives of real people no less than questions of global strategy, and Stalin did not take them all. Aside from the laconic Politburo protocols and the spotty memoir evidence, we now have a unique set of sources for the 1930s shedding light on decisionmaking in the inner circle: the correspondence between Stalin and L.M. Kaganovich while the former was on his lengthy annual holidays in the south. During Stalin’s absences, Kaganovich as tacit Second Secretary of the Central Committee, supervised decision-making in Moscow while in communication with Stalin…
On the face of it, it seems quite remarkable that a micromanaging dictator would absent himself for three months per year to a faraway place with no telephone during what had become the most crucial season of all for the Soviet economy: harvest time. One cannot imagine a British prime minister or American president so absenting her/himself, with or without a telephone…
Looking closely at one of these periods can be quite revealing.1934 was the last period of Stalin’s absence without a telephone. It was also the busiest year of the 1930s for Politburo resolutions: there were 3,945 decisions listed on Politburo protocols for that year and the Politburo met forty-six times. During Stalin’s holiday (August through October, 1934), more than a quarter of Politburo decisions (1,038 of the year’s 3,945) were registered and sixteen of the Politburo’s forty-six meetings took place without Stalin’s presence…
Politburo members took a large number of decisions without Stalin’s participation. Stalin intervened in only 119 (11 per cent) of the 1,038 recorded Politburo decisions taken during his vacation in 1934. The great majority of his interventions (91 of 119, or 76 per cent) were responses to initiatives from Kaganovich. The remainder consists of points first raised by Stalin. These numbers show that of all Politburo decisions taken in these three months, Stalin either did not respond to, or routinely confirmed, his lieutenants’ decisions 96 per cent of the time. Of his replies to Kaganovich’s requests for guidance, he confirmed his lieutenants’ proposal or decision without modification 84 per cent of the time…
Stalin left many matters to Kaganovich and the other Politburo members for decision, and many of them were not trivial…
On some very important questions, Stalin contented himself with providing general guidance or exhortation and then turning the matter over to Kaganovich and the team…
He often seems to have delegated more in the 1930s than previously. In September 1933, he wrote from his holiday location to Kaganovich and the Politburo in Moscow: ‘I cannot and should not have to decide any and all questions that animate the Politburo … you yourselves can consider things and work them out.’
More often than not Stalin went along with the party line. IIRC, he was regarded by his constituents for his opinion on matters because it near-always reflected the will of the proletariat, in the sense that their interests - the development of expanding socialism - were always at heart. More or less it is after Khrushchev that capitalism was restored in the Soviet Union, leading to its imperialism on the countries of the Warsaw Pact (in which resources were harvested from these countries but without an adequate exchange of goods).
As for the purges, it should be recognized that at the time there was a spectre of war looming over the Soviet Union. Many acknowledged that it was only a matter of time before another foreign enemy launched an invasion. As such, it was absolutely necessary to prepare the young republic for a defense, which meant removing its reactionary elements. Remember, at the time of the Great Purge there were still groups sabotaging and bombing factories and industrial machines - Bukharin was involved in a plot to depose/kill Stalin and revert the economy! To an extent, the stability and success of the Stalin-era USSR can be seen in its victory over Nazi Germany, as that is essentially the conflict the whole country was engaged in preparing for over the course of most of the 30's.
Guess I'll throw in a shout-out for /marx/. If you want anything clarified feel free to ask.