The sendline command treats <text> as though the user had typed it at the input line line.
If <text> begins with a slash (“/”), then <text> is taken to be a command. If <text> does not begin with a slash, then <text> is taken to be a message that should be sent to the current window's current target.
Ordinarily, when the user presses <enter>, that invokes the bind send_line operation, which throws on input. If you hook on input, you must use sendline to submit the input line for processing. If you don't do this, then the user would be unable to do anything at the input prompt!
The sendline command first appeared in ircII.