There's a variety of reasons.

Firstly you're in less of a rush, so when the stream becomes a dribble you just wait. If you're standing you're more inclined to get impatient and rush off.

Secondly, the stream is easy to control but the dribble is more likely to fall on the floor, so sitting avoids that.

Finally, pushing your gooch from our asshole to your nuts squeezes out the remainder. Its way easier to do this sitting.

Yes, I'm an old guy managing an enlarged prostate.