That's a good question.
WHO NEEDS? Its not a need for sure but race participants wants/likes to have their photos of a racing events.
There is a small demand say 5-10% of participants. But there is 1000 average participants in an event and some big events like standard chartered run even have 30,000 participants.
I'd also be very interested on how you derived the 5-10% of participants being interested to actually pay money for photos of a race or event.
It is based on a survey/study or an estimation plucked from the sky? Honestly, 5-10% is pretty high. You're saying 10,000 runners in a marathon, up to 1,000 would pay for photos?
In photo-biz, (or any biz actually....) one of the dangers is over-estimating the potential market and committing resources based on this estimation - it then becomes difficult to sustain and an uphill task to achieve good ROI.