Why are people running for charity?

This weekend there is yet another run for charity to help a young family with a husband dying of cancer – or a sick daughter, or a mother with a rare bone disease. There are corporate runs, runs for red cross, runs for homeless, runs for the hungry, etc. You name it – there’s a run for it.
Wouldn’t it make more sense, and be vastly more effective, for the participants to actually help the families, or charities directly? How about signing up to mow the lawn, or fix the rain gutters or paint the house, or get their car fixed? Something positive. Rather than exercising and pounding pavement, how about pounding nails and helping habitat for humanity repair a house. That helps the homeless.
I don’t get the connection between a 3 or 5 or 10k run and a charity. Do you?
I agree that even a run is better than sitting at home doing nothing. But is seems that there are other, more effective, ways to help those in need.
Do you participate in those types of runs? Do you think that is the best way to help the charity of your choice?