It's a good time for the company to attach various nondisclosure, noncompete, non disparagement clause(s) and they mark the severance pay as consideration. These agreements typically only hold up if there is commensurate consideration attached to them. This of course varies by state and local laws if they hold up at all.
IBM has been repeatedly sued, sometimes successfully, for age discrimination, so you can bet that severance package requires agreeing not to sue for age discrimination.
