It’s a gimmick. Like the ice bucket challenge, which was recently in the news again because the $115 million raised from the challenge funded research that ultimately led to the discovery of a new gene marker for ALS– a huge breakthrough.
People like to see other people get water dumped on them. It’s funny. That’s what gets them to open up their wallets. If not for the gimmick, $115 million would never have been raised in a single summer. People are happy to give tzedakah, they just need a reason to do it– the goofier, the better.
Of course, some walkathons or bikeathons are nice to support, because then you’re helping people get exercise and stay healthy. But I doubt most people contribute because of that. The “unrelated task” makes the contribution into something larger than just writing a check. It becomes a big event, a PR stunt. And it works.