As gravel bikes have become popular, so have bike-packing bag designs. Those bags that carry all the kit you'll need for a multi-day, self-supported ride.
Traditionally these would have been pannier bags on steel racks that would've attached to your bike. But this means both extra weight and the hassle of having to unbolt the pannier frames when you only wanted to ride for a few hours.
The other issue is that not all modern gravel bikes come with fitting points for pannier racks.