It's not to hold a properly tensioned belt in place but rather a method to get the proper tension on the belt. The benefit is that you don't need to pry against anything while you tighten the retaining bolts and the adjustment can be more exact.

Don

__________________
1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it