Helpfile suggests being larger and stronger will give you an oppurtunity for extra attacks in a combat round, and for them to lose some attacks. I've only ever used it on a felar and found it to be very rare maybe once in every ten attempts. I'd be curious to see how often/well it worked with a giant though.

Another reason to practice those is if as an A-P you want to use a spear as your unholy and get the Spine of the Akragala(sp) edge. Seems like a prereq is a skill % in those type of skills.