Longer cranks effectively give you higher gearing. If you consider that gearing is limited by the size of your outer cogs, then changing the crank might be something pros to gain a little extra torque (since they have stronger legs).

Or maybe 90% of pros just happen to have leg length that means they need something in the 172 range.

I can't imagine wanting to go much longer than 170 personally, since it decreases your clearance between your pedals and the ground during turns.