This one was for fashion/sales.

The original and higher end bikes it was for tyre clearance and creating a shorter effective rear stay, so tucking the wheel in more. You will see many with a bendy seat tube (see alpinestars)
It also kept the chain away from the stay.

There are lots of down sides

e-stay, comes from elevated chain stays.

Popularised in MTBs in the end of the 80s, but mainly early 90s became mainstream, then rapidly left the building.
Still seen with swingarm and various suspension setup, every now and again a rigid frame will come out.

Geometry doesn't change though.