They died so Poland and Europe live.
Very debatable statement if you look only at the situation on the front back then.
In hindsight, however, this irrational, f*cked up and extremely costly show of crazy, berserk bravery was noted and remembered by the Soviet Union. Perhaps that's what saved us from the Soviet intervention in 1981, similar to 1956 Hungary or 1968 Czechoslovakia. They knew that we are totally f*cked up in our heads and that we would fight with all our strength, so they decided against it.
Also, it served as one of many ingredients of Polish national mythos. Something that distinguishes us from the Czechs, for example.
Was it worth the deaths of 200,000 civilians and razing Warsaw to the ground? I can't answer this question...