The British Rail Class xxx thing is, as with many things on Wikipedia, a compromise that was hard won against some very entrenched viewpoints. The main argument for it is that it simplifies finding content for non-subject-expert readers (which is by far the majority of readers on Wikipedia) ( British Rail Class xxx is actually more correctly British TOPS Class XXX, but you would only search for that if you knew what TOPS was). The other advantage is that it's basically the least-worst option, suggested alternatives were to list them by owner (so search for the ROSCO name for modern classes), the operator (so separate articles for all the HST operators or manufacturer (one article for all the various European operators of EMD Series 66 locos then). Lots of people argued for lots of different things, and eventually a consensus was found, resulting in all BR-era onwards stock being categorised as British Rail Class xxx.
Every time a new class is announced this argument comes up, these days often with a very short "stop it" from a more senior / more experienced member. Go have a poke around the talk pages of the various classes if you want to see the arguments people make.