If you are only able to get 200 pounds past the sticking point, but your chest muscles are capable of handling 300 pounds once you're past it, you will be limiting your ultimate muscle mass and strength levels by only using 200 pounds.
This is where partial training comes in. By working only in the range of motion ABOVE the sticking point, you can use far more weight and, therefore, work your muscles at a far higher threshold.
By working at a higher weight, you will not only be getting your muscles used to handling those heavier weights, you will be building your connective tissue to help handle those loads.
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