[James2001]

I read somewhere that part of it was because of the transition between 4:3 and 16:9 and the blocks being in the proper shape would show that something was going out properly. Not that there weren't some shows which had the logo in the wrong aspect ratio, and of course that switch is long over, by 20-odd years.

I guess it also helps re-inforce the BBC branding, especially when these shows are sold internationally or on non-BBC channels and streaming (though of course it appears on countless shows that are never going to be seen outside the UK or the BBC's own channels).