It's great you're studying musicals' structure! Are you a Musical Theater university student? Either way, there's a lot of great learning ahead. :)
There are a few big things to think about here:
For starters, every piece of dramatic literature has a protagonist (aka 'the lead' of the story) who WANTS something. It's one of the crucial elements of any story. Early on, musical theater pioneers decided that a protagonist's ultimate desire can make for a very compelling song, and therefore, the classic "I Want" song was born. Despite countless exceptions, the 'standard template' of an "I Want" song will locate the song very early in the first act. It's most often the 2nd song of the show after an opening number. And traditionally, an "I Want" song is sung by the protagonist.
But sometimes, musicals don't give the protagonist an "I Want" song. Sometimes they identify the character's greatest desire within the spoken dialogue of a show.
>>> to be clear, there are any number of other musical theater songs where a character tells us what they want. But if we're sticking to the mutually understood standards of the genre, there are two components to an "I want" song: it's sung by the protagonist, and it's early in the first act.