Well, do some simple math. At 133MHz, on cycle takes 7.5ns, so cas2 PC2100 will respond in 15ns. Now, at 166MHz on cycle takes 6ns, so Cas2 PC2700 would respond in only 12ns. That 3ns faster response time is enough for a roughly 1-3% gain in many programs.
<font color=blue>You're posting in a forum with class. It may be third class, but it's still class!</font color=blue>