Finding a purpose is usually very personal to the people who are looking to organise a hackathon. It can be to empower your local community, targeting a specific section of the community, empowering a certain social group, making people aware about a different technology or the computer science in general or even just building out the culture of hackathons in your school or organisation. When you have the clarity behind what you actually need to do, you will be easily making decisions on how you need to implement it.