What were the uses you think about this fonctionnality mouser ?
some plugins would be creating entries that aren't real files, and so they would hande their own launches to decide what to do when user clicks.
But as for handling a result that the plugin did not itself create, it would be useful for a plugin that worked like a modifier and didn't create results of its own.
For example, imagine a plugin that was designed to run a program under a different users permissions (like admin), so you type your search and then type +runas:admin
then when you hit enter to launch a program, that plugin would see the +runas: modifier and take over the launching.