The primary incentive that comes to mind is improved availability. Often, instances can become slow, so I use another. By hosting a local instance I could always have a smooth experience.
Scores are federated, resulting in a consistent global feed across instances and a lack of uniqueness for each instance. I wish hosting an instance provided a more customized experience like this. It would be a great incentive.
If you host your own instance you are your own admin. That gives you personal control over content and settings. However, you need to pay for a domain name and you need a 24/7 server so there’s some expense involved. Then there’s maintenance like software updates and user needs if you take them on.
I think most admins take on an instance simply to contribute in building the Fediverse and create something to take some pride in. You get some clout in the community for doing that.