Petra
Cult Mother and Simpering Cunt
While I wouldn't use a rack for curls, there is a benefit in that a the larger in circumference the bar's handle is the more it builds hand strength and helps build grip....and the thinking goes that if you are already lifting that weight for something else and you have the grip strength to hold on to it there isn't much of a reason to not use a thicker bar to keep building that strength.
Of course most people that do it are probably not thinking about that or care about grip strength and are just doing it to do it.
I think most guys in the gym tend to do it for 1 or 3 reasons.
1. To get big arms. If you're body building, yea, you have to do it. For strength? You'll get it from deads, rows, pulldowns, chin ups, push ups, etc.
2. To impress the chicks. Yea, ask those 17 year olds how well that worked.
3. They're under the impression it's one of the core workouts.
In the end, I don't think there's any harm to it. But I rather personally not waste my time. I can work on improving my grip by trying not to resort to the straps on my deads.