This is in the context of Litvinov's policy of 'collective security', whereby Nazi Germany should be constrained by a network of multilateral alliances and commitments in Europe. Any German territorial expansion would therefore be deterred by the prospect of a joint response from the League of Nation powers (ie France, Britain and the Soviet Union).

Despite some progress (eg the Franco-Soviet Pact of 1935) there were a host of problems that ultimately doomed the policy. One of these was Poland. The Red Army was absolutely key to any anti-German alliance in Central/Eastern Europe but, as a glance at the map shows, Poland lay between Germany and the USSR. Soviet soldiers needed to be able to reach the German borders if any practical anti-Nazi alliance between the West and Moscow was to be feasible. That's the fundamental reason there.

Of course the Poles naturally had grave reservations about letting Stalin station soldiers on their soil. This came to a head in 1939, as the reality of war loomed closer, and Józef Beck repeatedly rebuffed British and French requests to allow this. Like much else in that summer, the scenes became slightly farcical: French diplomats tried to sign off transit rights without consulting Warsaw but Stalin insisted that the Poles themselves confirm this.

Ultimately the negotiations broke down, partly over the issue of Poland, and Stalin was able to reach a deal with Hitler. By the time those two went to war Poland was unfortunately no longer an issue.