The purpose of port knocking is for a remote host to provide authorization such that the server will open a firewall port (or, occasionally, start a server listening) when it receives a series of network probes. In theory, this can even be used with high-entropy keys to provide full authentication, though in practice that isn't usually done. Actually, port knocking in general is quite obscure; in my 12 years of work in the field, I've only encountered it "in the field" once.
In your case, your firewall port is already open for authorized hosts (authorization presumably determined by IP address), and the server is presumably always listening, so there's no use for port knocking. It would only weaken your security, as it would provide a vector for an unauthorized host to potentially get an open firewall port, if they managed to guess (or capture and replay) the knock. So, both useless and potentially harmful!
The scenario closest to yours where port knocking might be useful is if you didn't have a way to reliably recognize the authorized hosts (e.g. if they don't have consistent IP addresses), and you wanted to provide a way for them to get the SSH port temporarily opened for whatever their current IP happened to be. It doesn't sound like that applies to you currently, but perhaps in the future you will want such functionality.