What are the advantages of bikepacking bags? It seems that bikepacking bags are all the rage these days (frame bags, handlebar rolls, big saddle packs). 

To me it seems that they are prone to moving around and getting in the way, and it also seems that they make the centre of gravity very high. I can understand it if using a bike without rack mounts, but otherwise - what is the advantage of using these things over a traditional rack and panniers?
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.