Or a week off, paid vacation, etc, etc. As in pay someone 74.5k and give them 5 personal days instead of 75k/3 personal days. There's no incentive if the tax rate changes at 250k. But, if there are multi-tier tax levels, it's a lot easier to create incentives to take 500 less a year.
And, if your argument is going to be "Yeah but you get 2 days less work from them", no one misses anyone for 2 days.