The reason most organizers want to hold a race is to raise money for their group.
The money may be used for scholarships, for school equipment, to sponsor a trip for a group, to give to a charity, or fund a trip for a deserving person. The money raised may stay within the city itself, or it may be sent to a world organization to help people in a foreign country.
The thought behind this is that for a few hours work on a Saturday morning the organization can raise a nice profit for their cause. Some races can raise thousands of dollars for this cause.