Gathering emails is no small thing, but that's probably not the only motivation.
I haven't tried a pre-order, but the anecdotes I've heard suggests it works better for authors with large fan bases. If there are a fair number of people clamoring for the next book, a pre-order might be a mechanism for generating more buzz.
The way Amazon does pre-orders, with the sales recorded when someone pre-orders instead of on release day, doesn't give a book as much of big jump when it's released, but it does enable it to build a ranking before release day, whatever that may be worth these days. I would assume it also enable also-boughts to populate, which probably helps as well.