Reaching the 99% value will allow you to be more certain that you have reached the correct answer, when you see an ASSR recorded at a particular level with a particular stimulus. To reach a higher degree of certainty requires more time – this is the time needed to acquire more data i.e. keep averaging to reduce the residual noise and increase the signal-to-noise ratio.
One of the key advantages of ASSR is the statistical approach to interpration of the data, removing tester bias. However, statistics themselves are at risk of error and the 99% value minimises this risk. However, most people would agree that even a 95% certainty regarding the response is still high, and this criteria will be met in a shorter period of time.