Most research funders require outputs arising from research they fund be made available on an Open Access basis.
More people having access to your research, means more opportunities to form new research collaborations.
Greater control over what you and other people can do with your own research outputs (subscription-based publishing often requires you to sign away all copyright in your outputs, limiting what you can do with them).
Benefits for readers and wider society
Those outside of academia have greater access to research such as practitioners, policy makers, charities, small businesses and independent researchers.