If its in the contract, then it is instead of a tip. So you know exactly how much it will be and you don't have to worry about tip money on the day of. Its pretty smart. Usually more than people tip, but everyone has to make a living and it means you're not surprised or playing a guessing game.
Mine has gratuity as well which I think is a good thing, then I don't need to worry about should I tip individuals, should I tip the venue as a whole, how much do I tip...all the questions! It is in and done and I don't need to fuss about it.
Yes exactly what Louise said. I have it on mine as well! The venue should explain every little thing on there so you understand what it is you are paying for and if there are things you don't understand - ask them to make sure there are no hidden payments. Gratuity is common though.
Yes, if it is written into your contract you will have to pay it. Most venues do this so they can tip/pay their employees for the night. It's common courtesy especially if it a venue that is doing everything for you such as organising and catering.