What are the advantages of bikepacking bags?
They usually keep the bike narrow, suitable for trail riding where branches might snag panniers.

They use soft attachments (usually), so extreme vibration doesn't make undue noise or wear things out.  There are people with damaged racks simply from plastic pannier clips bumping along on them for months on end.

Cheap "bikepacking" (with scare quotes) bags are cheaper than cheap panniers plus cheap racks.

Bikepacking bags also help packrats reduce their load, simply by being more constrained than large panniers.

They're also suitable for rental bikes, when you're away from home.