The place of coronations of English (and then British) kings since 1066 (William the Conqueror).
Royal Tomb — monarchs are buried here, including Elizabeth I, Mary Stuart, Henry VII.
The national pantheon — in addition to kings, scientists (Newton, Darwin), politicians, and musicians rest here.
Well, we were, we looked, just for once. And by the way, the whole joke is that when you walk in this little church inside, there are graves everywhere on the floor (grave slabs), respectively, people are buried there, but everyone doesn't care, everyone walks and tramples, nothing sacred at all. And of course a bunch of these... around - where would London be without them...