Because otherwise it doesn't come out right. In my method, I use a hair dryer (not as dangerous as it sounds) and if I think I have a pure white product that evaporated right away, and go easy on the hair dryer, it turns to goo. Maybe it's what mtgeezer said, I'm sure it is. But I know that the heat definitely assists in the purging process, and it seems really important in first initial heating.
Basically, I just blast butane through my rig like you would normally into a frozen pyrex dish, then once it evaporates into thick goo I hit it with a hair dryer, it's not going to spark fire at that this point. I've always used the hair dryer on deionize because thats what I started with and don't want to find out if it's what is making the difference and get a batch of goop. I make sure I don't get the hash too hot and I keep the hair dryer moving back and forth. The frozen dish will help regulate temp, so you don't have to worry so much. I feel the bottom of the pan for progress, and once it feels hot as shit, and it's hard to keep my hand on it, I pull it off. I figure that's roughly enough 130-140 because that is as hot as it's recommended. My tap water comes out at 140 on hot, so I've been burned enough to know what it feels like by now, I'm getting thicker skin too, lol. I think using heat just speeds it up and makes sure it was done completely. Because when I go easy on the hair dryer, I get oil, when I go heavy on it, I get honeycomb. I use the hairdryer until their are no more bubbles popping, and I'm having trouble finding any bubbles left.
Then I let the pan cool back down to room temp, and I pop it in the freezer for a little bit until it frosts over really good, 15-30 minutes, pull it out, scrape it up, and then just put it on a piece of parchment paper, leave it out uncovered for 2 days and you get something like this.
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I don't vacuum purge because I haven't needed too. I've made some damn fine products, and it's definitely a lot stronger than the dabs I was getting at the Cannabis Cup in LA. I think the fact that I'm at 5200 feet has a lot to do with it. My buddy does the same technique in the desert, and uses a vacuum purge, and still can't get it to look like mine.