Why put woofer on the floor? - Insignia Audio Players & Recorders

Because low frequency sound travels better lower to the ground. The effect if placed high up would be minimal

LF sound has more punch that regular speaker sound, it also caused more speaker movement and if placed high up could in theory pull itself from the shelf or mount.

You tube has some sub sound tests you could use to show this theory. or maybe the candle trick of putting a lighted candle in front of a sub when turned up a bit may convince you.

put a candle about 6 inch away from a sub and play some nice bass it will blow out the candle more than a regular speaker

You get better bass response if the speaker is against a flat surface of the room, like floor or wall. Even better if it's in a corner.