The proximity of the speaker and the floor has a tremendous effect on the bass response of any cabinet. The advantage of tilting while on the floor is that you get the floor coupling bass enhancement yet get to aim a bit more of the clear axis of the speaker at your head so you can hear it correctly. (but don't aim the center "beam" at your ears, ever) For bass players and bassy sounding guitarists, this floor coupling is a major issue and can be fine tuned for nailing the right bass response. For us Jerry-heads here with virtually no bass in the tone, it really isn't all that important to have the cab on the floor. Somewhere between aiming the speaker way too low and directly at the eardrums is the ideal target.
One of the best reasons to tilt an amp is so that the speaker NEVER points its narrow and deadly treble beam at a human. If it hits someone in the audience withint 50 feet or so, it can be brutal. If the beam hits the soundman, his ears will be confused and he'll struggle with cutting as much treble as he can with the channel EQ and totally screw up the tone of the guitar in the PA, or he'll simply take you completely out of the mix. By tilting just right, you can have the beam blast upward at a tilt so the beam kind of hits you in the gut or chest and then continues up toward the ceiling and never hitting the eardrums. Also, its good be crafty with the beam and also be very sure not to aim it at your vocal microphone. That can wreak havoc as well.
Personally, I like a 1-12" amp to be about 2' off the ground and aimed a bit upward and not at my vox mic. If it's a vertial 2-12" cab, then I like it to start about 1' off the ground with the same basic tilt. A 3-12" can sit on the floor and tilt up like Jerry did.
B
... and it's just like any other day that's ever been...
