Originally Posted by
velocity
It's motivation through numbers.
It will keep you riding do to peer pressure.
It can help identify weak spots in your training.
It can give you a reason to keep that power meter on your bike.
It can get you to meet people on bikes.
It can give you maps of rides you would not know existed around the hood/vicinity.
It can give you excuses why you sucked that day
V
Pretty much summed it up. When you say "I'm just going to take it easy today," suddenly you realize you're in a segment, and then you push yourself that much harder.