When I was in 5th grade I participated in a charity event called "Jump Rope For Heart". People donated a certain amount of money per rope jump. I found myself wondering why it was structured that way – after all, people didn’t really care whether I jumped or not.
Many charitable events are structured this way, though typically they involve public walking.
Why do they work this way? Why not ask for lump-sum donations rather than having a bunch of people dig fence post holes? Is it make-work bias? Is it the labor theory of value? Maybe instead my willingness to jump/walk or otherwise participate indicates my commitment to the cause and in some sense certifies the event? A band-wagon effect? Maybe the dollar amount per unit of effort (jump, miles walked, etc) is so low that it induces people to donate more money than they would otherwise?
Rather than paying somebody to do busy-work, why not instead pay people to do something productive, such as soliciting even more donations?
Consider publicity as the main scarcity a charity faces. If you elicit volunteers to walk, run or skip rope for you, those persons will talk up the activity — and the charity — to their friends, both ex ante and ex post. They’ll even wear your T-Shirt "Cystic Fibrosis Marathon."
Since most of the people are exercisers anyway, the charitable activity doesn’t cost them much on net. In fact the exercise is one way of expressing a greater commitment to the charity and may encourage subsequent donations. Commitment, of course, is not infinitely elastic in supply. So some of the person’s commitment may be transferred to the charity and away from the ideal of personal exercise. Counterintuitively, in the long run the person may end up less fit but more committed to the charity. In other words you’re paying with some of your health and discipline rather than with your money.