Mark, i agree, water retention can be attained by that method too. [ dont forget to post on Bodybuilding.com, they are asking for you for questions]Or you could just ask me about it.
You certainly don't have to do it that way but there are several reasons to give it a try. As alex2363 says it can keep the bristles a little stiffer for better control if you have a high-loft or floppy brush. But the main reason I do it is for water retention. Holding the brush by the bristle base keeps more water in the deep center of the brush (the "breach") where all the heavy lifting takes place for mixing water with cream/soap. In other words I've found I can get a better lather by holding a brush this way. Give it a try and see how you like it. If it doesn't make a difference for you then just go back to how ever you were holding it.