It calibrates the gamma so it will match certain color standards like sRGB. It is more for color proofing so your prints will better match what you see on your screen.
If the monitor sucks, it will look better but it is not going to turn a $100 monitor into a $1000 one.
If the monitor sucks, it will look better but it is not going to turn a $100 monitor into a $1000 one.