Refer to Point 13 of U.S. President Woodrow Wilson's 1918 "14 Points" peace proposal. This point, which appealed to the large Polish-American community, stated:

An independent Polish state should be erected which should include the territories inhabited by indisputably Polish populations, which should be assured a free and secure access to the sea, and whose political and economic independence and territorial integrity should be guaranteed by international covenant.

I've emphasized the relevant sections of that point. The "Polish Corridor" was the simplest way for the newly independent Poland to get sea access while minimizing the number of Germans within the new country.

Having a seaport was deemed essential if Poland was to be an economically viable state. The question then became where to give Poland that sea access?

Even after WWI, East Prussia remained predominantly German. Danzig itself was almost entirely German, so as a sop to German demands, it was made a free city governed by the League of Nations and not handed over to Poland proper. The corridor was narrow by design: just 20 to 70 miles wide in order to limit the number of Germans in Poland.

This was a compromise that pleased no one. To the Germans, Danzig was under Polish rule. For the Poles, having their only seaport inhabited by an unfriendly majority (at best) and an outright hostile one at worst was intolerable. During the interwar years, Poland constructed a new port at Gdynia next to Danzig but entirely within the Polish Corridor.

This further stressed ties with Danzig, which saw its economy suffer as shipping traffic was diverted to the new port. After considerable acrimony, Poland compromised with Danzig and agreed to split shipping tonnage through each port.