Mainly control. You can set your own policies, such as deciding which ban lists to use. I have a backup server in another continent. It’s far less likely to have my account deacitivated for a mysterious reason by (both) my hosting providers than some email hosting company. So I feel that my access to email is more assured. I can have an infinity of ad-hoc email addresses and set any routing, or run any program on the server, based on the recipient address.