Question

explain why do the busses have a place to keep luggage under the seat

Answer 1

Because due to inertia of motion...luggage if kept on the roof will fall down without someone noticing it...but if it is kept it will be safe.

Answer 2

Almost all the buses have enough space to keep the luggage under your seat because - 
1. It might contain valuables, which may be too expensive to keep on the over head storage. Since under the seat storage has limited accessibility, the chances of grab and run are almost none to really small.

2.  Some people travel with food carry ons containing liquid or oils. Not keeping it under your seat might result in liquids leaking and ruining your clothes.