There's a legal distinction which impacts whether or not the employee is eligible for unemployment benefits, among other things.
(although legally "firing" vs "layoff" is irrelevant, depending on the situation you can get benefits despite being what a layperson considered quitting [1]).
[1]: https://www.darwingray.com/from-desk-to-departure-when-movin...
There's a legal distinction which impacts whether or not the employee is eligible for unemployment benefits, among other things.