Allan's right. The vang is also used for going upwind. We have mid-boom sheeting, too. A vang is one of the key sail shaping tools and is never redundant.
I have made the point in the past, however, that for those with end boom sheeting and, perhaps, small travelers at the transom, it is easier for those skippers to dump the mainsheet in a puff than their traveler. On my boat, and on your 42, dumping the traveler is much easier than dumping the mainsheet, because pulling in a lot of mainsheet is a LOT more work than just pulling the traveler back up.
Depending on wind strength, the vang on a broad reach can be used, as Allan says, to either tighten or open up the top of the sail, even if the traveler is all the way to windward.
I recommend two things:
1. Practice
2. Buy Don Guillette's Sail Trim Book and Chart - very good material.