The #4 aux pump provides brake pressure without needing a running engine or APU. Handy for all sorts of reasons. The optional #1 aux pump allows for body gear steering in the same configuration. In plain English, an airplane with both #1 and #4 aux pumps installed can be towed with just a GPU plugged in.
It does not have to do with bleed demands on the APU (MEX excepted!). As noted, not all airplanes have the #1 aux installed and the MEL allows the #4 to be inop; the end result then being that the push back and engine starts are done with the #1 and #4 demand pumps working via the APU bleed.
Fair enough, I didn't consider towing too much since I'm rarely, if ever, there for that, but it makes sense. The aux pumps are on the ground handling bus so I guess there's not much else they could be used for anyway. Thanks again for the responses.