In XCOM:Enemy Unknown and XCOM:Enemy Within, you have the option each turn to make two limited-distance moves. Alternately, you can make one double-distance move ("Dashing").
It seems like splitting the move into two parts gives you more options. If you make a half move, and happen to find that you've flanked an alien you couldn't see before, you can fire on him. Or flee. If not, you can make the other half move, and wind up in the same situation.
I thought it might have to do with enemies using overwatch, but it seems like they can fire on you when you move through thier line of sight either way?
So, why would you dash, effectively losing the mid-move option? (I've considered that it's for people too lazy to tap twice, but set theory suggests that highly impatient types may not have huge overlap with XCOM players.)
Note: This question touches on the pros and cons of dashing, only in the context of noise, and doesn't speak to the potential benefits of dashing vs. splitting the moves.
Best Answer
Mainly, dashing grants a defensive bonus (-20 Aim) to overwatch shots.
I've used it rarely when I need to soak an overwatch with a rookie, to get my shotgunner or heavy into a better position before firing.
Another sometimes benefit, is that it will take the absolute shortest path to get to that location. This will allow you to get maximum movement without exactly positioning your middle move. Be careful that this doesn't dash you through hazardous terrain of course.