Why do the Jonas Brothers wear rings?

They are called Purity Rings which they wear to show that they have made a promise to God, and themselves, to not have premarital sex (have sex before they are married). The brothers are well known for wearing purity rings, and the boys are good pure christian men, so they started wearing them after their mother suggested them to.

A purity ring is a statement that the wearer will not have sex before marriage. This is meant to show their commitment as a pure band. The purity rings are there for multiple reasons. One of them being the fact that their parents are very devout Christians, and want them to wait. The next as a marketing technique used to sell them as a pure band, so children's parents will accept them.