Rear shocks absorb the power of acceleration, and balance the bike through the turns.  Without the proper spring rate, and without adequate damping characteristics, your rear shock simply won't allow you to go any faster.  The more you push the limits of a stock shock,  the more you chew up tires and risk crashing.  Aftermarket shocks give you the rebound and compression adjustability you need, including ride height to allow for changes in tire size and manufacture.  With the right spring rate on a high performance shock, and the right settings, your no longer limited by anything but your own desire to go faster.