You need to establish back story or other exposition, you need to provide a motive, you need to establish that named characters can die, you need it to cut off options so that the characters have to go to the climax. . . .
And boy is it a nuisance. Particularly if you're out of sorts and don't particularly want to write. It's hard to catch fire on those scenes. The habit of switching stories makes it worse, enough to make some stories sit on the backburner for months on end.
And sometimes, of course, the problem is that it has to be hacked out and taken from another angle. Hard to judge. sigh