In the "have you ever noticed..." category, have you ever noticed that for about a decade now it's been almost mandatory for television shows to start with a teaser (also known as a "cold open") -- a short segment before the opening titles?
It used to be much more common for a show to begin with the main title, then go to a commercial, and then start the episode proper when it came back from the commercial break. If you look at a typical show from the '60s or '70s or '80s, the only thing before the main title is a 30-second highlight reel of clips from the episode you're about to see. ("Moonlighting" parodied this practice in one episode by prefacing the main titles with a bunch of randomly-selected, nonsensically-arranged clips from that episode.) Or it might do what "The Flintstones" did and start the episode with one clip of a scene that would be seen later in the show. Or it might just skip the clips and go straight to the opening theme song.
There were of course plenty of shows that used teasers: "Star Trek" did, and many of the Screen Gems shows did, and "Barney Miller" did. But it wasn't considered de rigeur, and many shows preferred to do a short segment after the opening titles rather than before. ("The Dick Van Dyke Show" briefly experimented with opening "cold" and going to the main titles, but they quickly switched back to the format of having the main titles, followed by a short scene, followed by a commercial.) Even by the '80s, most shows didn't have anything before the opening credits unless there was some kind of gimmicky reason for having a teaser: "Hill Street Blues" used a teaser to incorporate the "roll call" introductions, and "Cheers" used a teaser to start the show with a little gag that didn't have anything to do with the episode proper.
At some point in the '90s, though, it became absolutely mandatory for U.S. network shows to start a show "cold," saving the opening titles until after the first scene. Just about the only shows that didn't do teasers at that point were animated shows ("The Simpsons," "King of the Hill"). For drama shows, the format is generally a recap of previous events, followed by the teaser, followed by a very short main title.
As to why this happened, I guess it's all part of the networks' increasing obsession with not letting viewers switch to one of the (many) other channels: the theory is that if the end of one show isn't immediately followed by the beginning of the next episode, viewers may let their attention wander and change the channel. I don't know that this theory always holds true, because sometimes a show may have an easier time hooking viewers with a distinctive main title than with an expository opening scene. I sometimes wonder if "Veronica Mars" wouldn't get more people interested if it hit them up-front with the title sequence (which makes it clear that this is a cool show about a teenage girl detective) instead of starting with another recap followed by another piece of Veronica's voice-over narration. The teaser for "Hill Street Blues" worked because it was something interesting and unique that could pull in viewers while establishing a framework for the episode; a lot of teasers are just confusing to new viewers, because there's been nothing to establish what the show is about. A good compromise solution might be the "Law and Order" way: tell the audience what the show's about, then do the teaser, and then do the opening title.
I will note that the supremacy of the teaser seems to be lessening a little bit these days; "Entourage" doesn't do teasers, and neither did "Arrested Development."