It was weird. I just finished a hoplite vs. hoplite custom battle and I noticed that when I had my units attack in guard mode, they were pushing foward rather than stepping to the right. Phalanxes actually gaining ground! And out of guard they stepped to the side counter clockwise. This is nice if I've stumbled on to something.
I found it useful using the pushing in guard to force enemy units who were too deep into my line. Being out of guard mode is good when say you're losing on one part of your line and you need to kill quickly or say you want to have your rightmost unit flank the enemy because of the natural rotation that's been mentioned in other posts.

The only down side I see is that you kill less in guard mode so in one on one unit confrontations, you'll get killed more often.