What can be done with a leaked rsa key?

I’m new to hacking and I’d like to get better at it. Let’s say that I’ve managed to leak and decrypt a website’s rsa private key. What could I do with it? I’ve found this command on google but it returns me “host key verification failed”

ssh -o BatchMode=yes -i “rsa key path” root@“target ip”

What’s wrong with it? Thanks for the help

chmod 400 rsa_key
ssh -i rsa_key user@ip
you need to know the user

Thank you very much