The way i see it is that it helps line Tony's pocket so that he can afford to keep BCUK running so that when you next come back for assistance/help/ideas etc the forum is still running.
There have also been a number of kit competitions and special offers that have only been open to full members and you get a free sew on BCUK badge too.
Basically its a contribution to the running costs of the site and for that you get rewarded with a number of opportunities that arent always available to everyone else.