It shows you have a genuine interest in a subject which is hugely important when applying for university. You also get a feel for the course itself so it'll help decide if you really do want to study it for at least another three years.
Yeah i went on a Debate Chamber one and it wasn't really beneficial to me in terms of learning about my subject. I included my attendance on my personal statement and I did get 5 offers though, so maybe if anything it shows your commitment (You know, the fact that you're paying a stupid amount of money to waste a week of your summer holidays)