Employers give it for one of three reasons:

1. It was negotiated as part of your job offer. (It is too late to try to negotiate it at any other time.)

2. It provides a lever for future behavior. "By accepting this severance payment, you agree not to sue THE COMPANY for any reason, and not to disparage THE COMPANY, and ..."

3. It makes the employer look reasonable to current and future employees. "Laid off 11% of the workplace due to economic conditions; 3 months of severance and various benefits" is much more conducive to hiring than "Laid off 11% of the workforce with no notice, no severance, no nothing".