Based on my (limited) knowledge, I believe the raising of temps through primary is done for two basic reasons. The first is attenuation. Keeping the temps on the rise will keep the yeast active so they finish up the job instead of floccing out early. According to a Trappist brewer interviewed in Brew Like a Monk, some strains will do this quickly and permanently if the temps drop enough (rather than raise).
The other reason is to allow the yeast to clean up post fermentation (this is also related to keeping them active, I suppose it could be viewed as another effect of doing so, as with the attenuation issue above). Even though you might ferment at high temps, into the 80s with some of these Belgian strains, if you keep the temps going up throughout, you will still end up with a pleasant ester profile. I'm wanting to say the yeast tend to process some of the less desirable esters more, but I could be imagining this.
So if you want to keep your yeast happy, try to keep temps steady or steadily rising throughout fermentation. Don't let them drop until several days after you have reached your FG. Even if you let things get out of control, and rise up to 85F or whatever, chances are it will be better to keep things at 85 until it finishes rather than drop it down to 75 in an effort to "save" it. I'm sure there's more to it than this, but I'd be lying if I claimed to know much more.
