These deals vary but, as the name suggests, you get cash – in addition to the money you’re going to be borrowing. You may use it to pay for moving costs and furniture etc.
Cashback deals are perhaps best seen as a sales technique to get you to take out a mortgage with a particular lender.
If you need cash it may be an idea to shop around to look for better deals from your bank, credit card etc. (Best not try the local loan shark though).