Because of the non-fiction nature of the show, mere 'extras' were able to be cast with names, personalities, and real consistency, so that subconsciously their faces had soaked into my subconscious. It made that universe real.
It's like school years, when you really only knew your friends and teachers by name, but as you walked the hallways, you could recognise with some familiarity all the unknown faces. If you were to go to a new school, yes everyone there was strange... but they weren't your strangers.
Background characters, if visually interesting and consistent, breed sub-conscious familiarity and depth in the world.
Now, I'm not talking about a large supporting cast like The Simpsons - think more Nurse Kelly from M*A*S*H. Unlike a supporting character, she never had her own Episode, theme, back story or even a full conversation - but she was consistently there, in the background. I like the idea of making an audience feel like there's a world beyond the 'Regulars'.
And of course, if as a writer you need someone new down the track, you already have a range of familiar characters, warmed up and ready to go.
Why, after watching the first Season or two of Voyager, there were times where I bet they wished there was a more interesting cast of 'nobodies' to draw upon. You always knew when there was an Episode coming up about a 'new Ensign', as they'd start setting them up two or three episodes before. It was always really obvious, as normal episodes would lead you to believe that there might only be ten people on the whole ship, and suddenly here was Ensign Ricky having his 15 minutes. But hey, at least they set them up.
Personally, I think there is some untapped potential using the idea of The Consistent Extra in animation, where I really haven't seen it used that much.
I just have to make sure I use it in a world where it is actually likely that characters will reoccur.