Many strong employees are ambitious and need the prospect of progression to be happy with their job. When growth slows below the employee ambition rate, not everyone can be sustainably paid more. Then you're faced with a choice of removing people to make space or employees being unhappy.

You can let this equillibrate naturally. The employees who are both ambitious and skilled enough to land other jobs will disproportionately be first out the door, leaving room for others. But this reduces the average talent level, and hurts company prospects, so many companies choose other paths. Some proactively fire the bottom 15% every year. Some heavily differentiate pay to make up for lack of promotions. Some constantly reorg and fail to find spots for employees in internal interviews. Offering a paid out for those who were thinking about it anyway seems a uniquely humane solution.