It can serve a lot of purposes, including all of those mentioned in other answers.
One other thing I'd like to mention is regarding the timing of the shoot. I've done lots of "engagement shoots", where couples are looking to test the waters and see how we get on, ask questions, and check out the results before they make their decision.
But I've also often done "pre-wedding" shoots with couples who have already booked with me, and these we sometimes do just a few weeks before the wedding. They are a great way to reconfirm everything we've discussed previously, as well as to talk about poses, locations, options for wet weather, what the venue looks like at that time of year (some, like wineries, can look VERY different depending on the time of the year).
So it's worth considering an engagement shoot if you're still choosing your wedding snapper, or a pre-wedding shoot closer to the date - both can be very helpful to clients as well as photographers.
Thanks for the great question! :)