Idler arm: A pivot point in a parallelogram steering system that follows the motions of the pitman arm. A worn idler arm bushing typically causes steering wander (looseness) and toe wear.
(I cheated, this comes from about.com or abooot.com as our US friends would have us say)
If you look at this pic, you can imagine the weak point under stress -- that is, at the very bottom there is not much support. The brace adds, um, some bracing at the bottom which hopefully will keep all those parts healthy and happy.
In the end it's because of running with bigger tires over bigger stuff than parking lot speed bumps.
