How exactly is this not just people being fired? I understand they get severance so that's nice for employees, but what's the goal of doing it this way from the companies perspective?
A firing usually means the employee did something wrong, such as harassing a co-worker, stealing money, or violating critical policies. A layoff means simply that your services are no longer needed, and that's it.
In addition being fired looks really bad. Being laid off not so much (if at all).
There can be legal differences as well as some of the reasons mentioned by others already.
Many states make it illegal to fire some or all types of employees without cause.