I think in part it’s a fashion statement like a lot of things in pop/top 40. sometimes it’s ‘the thing’ sometimes not. Outside pop music, some genres like Trance almost can’t have a defined ending as the while point is one big neverending trip.
And simply, sometimes a song ‘needs’ a beginning, middle and end point, sometimes. Think of it like a book. Sometimes the author says all there is to say and brings a story to a defined end. Sometimes it’s a cliffhanger on route to a series/sequel.
Agreed on trance.