Earlier quoted context omitted.
Anything benefiting from a static IP address, such as running your own VPN, mail server, Bitcoin node, TOR node... the latter of which got me banned by my bank's security team because I was marked as "suspicious traffic" (wasn't even an exit node) - preventing me from using online banking. Talking to support proved fruitless, however the ban was lifted as soon as I changed my IP address.
Note that the IP address is shared with other PIs and there are restrictions on which ports you can use: https://examesh.de/en/docs/colocation/accessing-the-pi/
Looks like web hosting or a mail server is completely out of the question.