Like any musical, there is always an opening number. The opening number can get you emotionally attached to the musical. They also have to make sense for the musical. Aladdin has one that is quite effective: Aladdin’s opening number, Arabian Nights, is quite effective. I will try to explain.
In Arabian Nights, it is sung by the Genie, one of the major characters. You get to know what Agrabah is. In so many ways, the opening number foreshadows different parts of the story. You are introduced the principal and supporting characters- even though you may not know their names yet- that is if you haven’t seen Aladdin.