Because watching TV isn't straight-forward enough
I used a list from Wikipedia, which has since been revised because the list represents (ONOZ!!) original research. (Lucky for me, they have that handy history thing.) Going into it, I wasn't sure how accurate the list was, but upon viewing, it mostly made sense as "story" order.
Actually, story order was pretty good up through Buffy5/Angel2. Episodes end up spaced out pretty evenly, and crossovers fall right into place. In fact, it was interesting seeing, say, Buffy leave Sunnydale to take care of something and have her pop up in LA right after.
But, once you hit Buffy6/Angel3, story order isn't all that it's cracked up to be, which makes a whole lotta sense (since "Buffy" was no longer on the same network as "Angel"). There's still a proper story order to things, but it's not particularly even. You start getting big blocks of one show or the other, and not entirely because the episodes bled into one another, each starting seconds after when the previous left off.
Buffy7/Angel4 was probably the worst in terms this story block thing, particularly towards the end of the seasons, since you have to have "Angel" finishing before "Buffy" and a lot of the episode bleeding, too.
The worst thing about the story blocks is that you leave one show for so long that you lose its momentum. A lot of Buffy7 momentum was lost by going back to Angel4's story. Then again, looking at the broadcast dates, Buffy7 had a series of three episodes aired in the span of about seven weeks, which is some serious momentum-killer.
Still, when it's all said and done, I think story order was a good choice. I may even do it again (at least for Buffy4/Angel1 and Buffy5/Angel2). Switching discs practically every. single. episode. wasn't my favorite thing ever, but that's okay. Getting a better sense of the stories, how they connect, was more than worth it.
I want to write out some thoughts on the two shows, but that'll be a little later. I've got some things I have to do first, plus my thoughts need to stew a bit longer; they're a little jumbled at the moment and not as clear as they should be.