I try and get as much use as I can out of mine. If you own a gown you have free entry to the castle. They're also lovely and warm on a winter's night. One tradition is to wear them to exams. I like to do it but not many others seem to which is a bit of a shame. Another instance of gown use I can think of is if you become a university ambassador.
I recommend The Sinner too. Sometimes on the first floor of the Student Union people post notices wanting to sell their gowns. In July I saw two gowns on sale in a charity shop for £30 each.
You by no means need a gown to get on but they are nice to have. If you are going to the gown sale during Freshers Week I really recommend turning up as early as you can and definitely in the morning. I once volunteered to sell gowns on the afternoon shift but I was surprised to find that my help wasn't needed because all the gowns had sold by then.
Get going with the all day revision thread