On some horizontal bars there also two bars positioned perpendicularly to the main bar. So when you grip them palms looking toward each other (as when you applaud). What training on such bars gives?
When you do a pull-up with your palms facing each other you are using a neutral grip. The standard pull-up is done with a pronated grip.
When you change the orientation of your fist you alter, slightly, which muscle groups get worked more. My understanding is that a neutral grip will work you forearm more and a pull-up ultimately works you lats much harder.
Stealing some verbiage from this website:
Pull-Ups – The most difficult of the 3 grips, the pull-up recruits the musculature of the upper back more than the musculature of the elbow flexors. For this reason, the pull-up grip is ideal for packing on upper back mass, but not ideal for arm growth.
Neutral Grip – This is the strongest of all pull-up grips. Parallel wrists place more emphasis on the brachialis, eliciting substantial arm growth. Taking advantage of the biomechanically advantageous grip also allows for utilizing heavier loads, producing greater recruitment of the inferior fibers of the Lats.
For your information, the third grip mentioned here is a supinated grip -- this is a chin-up.