Secondary Ferment. Temp with Belgians

So, I've been reading "Brew like a Monk," and coincidentally, I am also finishing up a 4 week primary fermentation on a Tripel. This weekend, I am planning on transferring to secondary, and leaving it in my guest bathroom bathtub (in a carboy!) for a couple months (temp stays at +/- 70*).

I've noticed a common theme in this book, where secondary fermentation temps are quite a bit lower (in the 30-60F range) than primary fermentation temps. What is the purpose of lower temps in a secondary fermentation? I understand how higher temps will bring out more banana and fruit in a primary fermentation, but how about in a secondary?

I think the higher temps in the primary are to induce yeast related flavors and aromas. Typically, you would start with a common ale temp and slowly raise it or let it naturally rise into the high 70's. or even 80's. This also allows the yeast to ferment the beer out to very dry conditions. The secondary would be a stage for settling particulate matter and conditioning, so the lower temps would promote that action.
I've had a Belgian golden and Chimay Tripel clone in the fridge now for several months and they are both doing very nicely having gone through this process.
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Beer Diary...

I actually just finished reading the chapter on Yeast in Brew Like a Monk. There is a lot of discussion on the starting primary fermentation temp, and raising it, just like you said.

That makes sense though - keep the secondary cool, because the yeast will fall to the bottom at lower temps. I'll give it a shot this weekend.