Basically, before treble existed manufacturers had to basically make each android update from scratch including updating all the drivers. this was annoying as shit and made updates slow
with treble, drivers exist in their own partition and the android OS can be updated independently of the drivers. it has greatly sped up updates in recent years
a side effect of this is that it lead to GSIs in modding communities, where the idea was you could have a single flashable OS image for every treble phone. in practice, it didn't work because it isn't a simple plug and play solution