The verse before the chorus typically serves as a transition between the verse and chorus, and is often used to build anticipation and momentum before the chorus hits. It may contain lyrics that introduce a new idea, reiterate the main theme of the song, or provide further insight into the story or emotion of the song.
The verse before chorus is not a mandatory element in a song structure, and some songs may not have one. However, when used effectively, it can enhance the overall structure and flow of the song.