It's a trend that started in America and I didn't realise how popular it was becoming until I learned that two acquaintences of mine had done one before their April nuptials last year. What is the point of such a thing one wonders? They're an extra expense, and what value do they really serve?
Sometimes you can get to the wedding planning and realise that you've not got a single image of the two of you together as a couple. You've been together for years, but somehow just not managed to ever get in the same frame as each other. It happens. An engagement shoot certainly corrects that problem.
It also gives you an opportunity to get to know how photographers and photography work. There's no pressure during an engagement shoot so you'll have plenty of time to ask questions: what does that do? Which positions look best? How do you make the most of the light? Think of it as a dress rehearsal for your big day. It's an added bonus if you're using the same photographer that you plan to use for your wedding, but it's not essential.
Remember: marriage is hard work, the wedding is just the beginning.