They need money to book events that people take part in both at fresher's and throughout the year (spring formal, Christmas dinner) and they also supply stuff. If you don't pay the fee at the beginning of the year then they take it from your deposit to cover it, and without it they wouldn't be able to book events for the halls and fresher's (especially since the fresher's is organised by the students at Reading). You could possibly get out of paying the fee if you speak to the SU and tell them that you will not be attending any events and are not using the service. As 95% of students attend events they make it an automatic thing otherwise people would attend everything and then not pay towards it leaving them out of pocket. I said it seems alright in the grand scheme of things because it saves them hassling me for money each time they want to do something, and all of my friends paid between £55-£100 for there's so I consider myself lucky that Readings hasn't made there's higher.
With the NUS card, I just use my student card to get discounts at places like clubs, restaurants, clothes shops etc.. I haven't had anyone ask for the NUS before as long as you have something says student on it, although the university itself may be more strict. I'm not going to bother getting one until I find a need.