If you recall from a post here several months ago, the U.S. government only utilizes 7% of its server capacity. Although it’s important to leave room to grow, wasting so many resources is costly.
It is also a bad idea to over-utilize a server. For most applications, you do not want to be using more than 70% of your CPU or memory on a regular basis. When you constantly find that intensive applications are chewing up 80%, it’s time to migrate to a server with more headroom.
Using too many server resources also causes instability and will crash a site in the event of a traffic spike. Finding the balance between too much hosting and too little is key.