An average suit makes you look... average. Your attire leaves no lasting impression and fails to set you apart. You think that because the suits are cheap, you can get a few and have variety. The downside is that now you have not one but several suits that don't do you any favors. You see yourself in the mirror and it doesn't inspire confidence. You go about your day looking and feeling average.
A great suit gets you noticed every time you wear it. You feel confident and successful. You look and feel sharp; different from all the other guys who look and feel lackluster. Even if you're only able to add one great suit to your wardrobe at a time, you've made the right decision.