If you stay at a company, they will give you some fixed pay increase, perhaps tied to your annual review. After a few years, your pay could get seriously out of whack with the market.
Many companies have a policy of only fixing such problems after the employee gets an external offer. Of course, by the time the employee gets the external offer, they are pissed at their current employer about being underpaid, so they leave.
tl;dr; pay your employees market rates, or plan for turnover of employees experienced in your own practices.