Well there are lot of reasons for which people run in marathon:
1. You feel motivated : when you run in marathon you feel energetic, motivated and help you to stick to your training scedule
2. Prize money: Many people run for reward this show how much involve you are in community work and promoting goodness
3. Improve health; Running will benefits you and this kind of marathon will actually be boon
4. Get and excuse for leave in the office: Yes this is truer that many corporate's promotes goodness and they will give you a leave for this purpose and even sponsor you
5. Opportunity to meet new people: This provide you the opportunity to know more people outside and increase your contact list