Some of the panel actions like advanced servers & players query, kicking, banning or muting a player, and executing commands require RCON to be enabled on your servers.
No, the plugin is optional and only required if you want to use the panel actions. You can still use the panel with the SimpleAdmin plugin, not that you must use either one of them.
Since the panel is made on top of node.js, it's not possible to run the panel on a web hosting, as web hosting only supports static websites (.html, .css, .js, etc. files).