Act 1 and act 2 meaning??

hi everyone im quite new to the Wicked community, i have watched WOZ and Wicked Part 1 obv bc part 2 isnt out yet, but my question is why is there an “ act 1 “ and “ act 2” or/and part 1 and part 2 why couldnt they make it in one act/part, and what is the importance meaning behind being it the first or second act? I know that there is a few years between the first and second act/part but i was wondering if there was more to it. Ty!