If you lay people off, you’re on the hook for their unemployment. A voluntary buyout avoids that, and also avoids mandatory reporting for layoffs.
That’s not entirely accurate. Washington State pays unemployment, not the company. The state funds unemployment by taxing payrolls, but it’s much more indirect than most people think.
Claims do affect the rate they pay next year, but the company isn’t “on the hook” for you personally, at least not any more than they are for your social security income.