I've read a bunch of stuff about do-it-yourself wraps on the web, and from the critiques, I don't think you get the same experience. Most of it was pretty negative. I guess if you have a willing partner they can cinch you up tight, but I don't see the same level of tightness of the wrap just doing it yourself.
I believe they really pull it tight because (supposedly) as the enzymes are liquifying the fat globs, it moves them around to the right areas. Maybe spreads them around while the rest is excreted.
That is a total layman explanation, but it makes sense to me.