When you reach top pay role model with no bonuses any more, where is the incentive to improve yourself or operations? Bonuses or stock could replace the lack of opportunity to earn raises at evaluation time. Imagine, you reach this point at 10 years and plan to work another 10 or so at the same pay -- it isn't realistic. Something has to be done to retain highly skilled workers who want to stay with the company.
Why is this important?
Being competitive in keeping trained associates and rewarding dedication. Role model work and service needs to be fairly compensated.