What kind of operation user can be made through ssh (when enabled) ?
Operation? Operation user? I'm not sure I understand. You can do virtually anything (and more) via SSH that you can via WHM. Some abilities aren't even available in WHM. Some abilities are more difficult by commandline, some are easier. I'm talking about if you have 'root' access via SSH. If you don't have root, you can still do many things via SSH that you can't do (or not as effectively) via the Cpanel interface. Usually, the only people that absolutely would require PHP are people who are site developers who want to be able to log in and edit files on the server in quick fashion instead of downloading/modifying/uploading. Also, people who are developing PHP / CGI applications, etc who would want to debug them - You can do that a lot easier via command line. This is true regardless of whether the user has a regular shell or a jailshell.
I do nearly all my site maintenance via SSH and an editor (such as vi or nano). I do nearly all of my server maintenance via SSH access (and just su to root).
Of course, 99.9999% of the people who have websites have no absolute need to have SSH access - and if you give it, then you could potentially open yourself up to additional problems with them attemptingi to hack your system - which would not be as easy for them to do if they did not have shell access.