I suppose the goal is to ensure, monit is able to report issue -> up and running and SMTP functionnal.
Let's suppose one of these two "dies" for any reason, an issue raises but you get never notified cause you are not aware that the system is down.
Having regular heartbeat from the monitoring system helps to know that it is up and running.
Again it just a personal guess but i can understand the need.
Like they says on some movies:
bodyguards: "we are going to protect you VIP."
VIP "Ok but who is gonna to protect you (bodyguard)"