Off-course, server monitoring tools are very helpful and easy to find the fault on the server. There are number of tools available in the market which can monitor the traffic on your server, exim service, get-post request to and from the server, mysql processes, disk space on the server .
It is really effective to use them because those have setting which can alert you by email or voice message so you can take quick action on server and avoid the more downtime of the server. If your server is going to stuck then you come to know before due to the monitoring too so main use of it is to avoid the downtime of various services on the server.
Feel free to configure them o your server.