If the employee doesn’t like the terms, they’ll just get the statutory minimum.
It’s pretty rare anyone takes that option though!
In most cases, an employer will want at least something in return for providing severance pay.
But NDA is another reason, or even keeping unemployment insurance premiums low by having everyone that accepts severance state that they are voluntarily leaving the company in exchange for the severance.
By voluntarily separating, they cannot collect unemployment benefits from the state, which means the state charged the employer less in unemployment insurance premiums.
But I suspect it’s mainly as a carrot to sign the NDA/non-compete/whatever else they want you to sign to keep not talk about what happened.
These kinds of things are not always about the strict minimum cost actions. In general, it is a better world with some degree of human consideration being part of things.
I am quite sure many of the cases I had involvement in could have been done cheaper. However, I am also quite sure the people involved had what they need to get through to their next gig with few worries too.
That is worth a lot and these days one can never know who may be working for or with who.
I hope you start a company and are in a position to give severance to fired employees and not make them sign non-competes.