The Olympic tricep bar will put your hands in a more favorable position while doing skull-crushers and other triceps extension movements. If your goal is to build big arms your focus should be on the triceps muscles. The triceps make up 2/3 of upper arm mass. Focusing your weight training efforts on them is what will give you the big muscular arms you are striving for.
The Tricep Bar also works great for hammer curls, another mass building arm exercise. The Hammer curl's main focus is on the forearms, but your bicep will also get some work also. This makes the tricep bar a great tool for building overall bigger, muscular arms.