Basically the benefit is 5-6 months of not paying for accommodation and food. That money can be put away so that when you get back to the UK for the 4-5 months (I think?) at CTC, you've got money for a deposit on somewhere and some in reserve to help cover your costs there. I've not started looking into prices within commuting distance but I'd imagine that after rent, any bills and food, you're not going to have a lot left over so anything you can put away to keep a safety net for your return, the better.
In terms of what I'm spending money on over here, it's toiletries, any food/drink (juice, biscuits, fruit etc) for in the room and any trips out and meals out/drinks.