Just why did they individually name all the episode parts back then? Anyone know? And when did that change?

Don't know why --- I suspect it may have had something to do with accounting and keeping track of how money was spent (Verity Lambert's commentaries make it sound like the BBC weren't willing to fully commit to keeping the show on in the initial stage) --- but I know they kept doing it at least through "The Space Museum" so, what?, a year and a half?