I only know about Nottingham, but i have been told by an admissions tutor that they tend to keep a small number of places open for UCAS Extra applicants for some courses. This is for 2 reasons. 1.) INTERNATIONALS!!!! Often the application is only open to intl students. The university tries to get as many as possible as they pay higher fees, in fact Notts has more intl students than any other uni (about 15,000) 2.) Try to improve quality. If the admissions tutor feels that the quality of applicants has not been as high as usual (Notts has had a big drop in applicant quality recently), then they will keep a small number of places open in case some AAAA students who got rejected from all their choices wants to choose that uni.
Also, dont forget that a lot of the bigger universities have more courses than smaller unis, and it is usually the more random variants that are on extra. Durham fills all its places as it only offers a few courses, whilst bigger unis have loads of weird ones like History with russian that attract few applicants.
A friend of mine applied though extra after being rejected by all of his choices, even though he got AAA, and received offers from Bristol, Warwick, York, Nottingham and Exeter for a whole variety of subjects, ranging from Archaeology, English Lit, Film and classics (he wanted to get offers to make him feel better so applied to loads and kept rejecting them to take the piss)