Not only do some companies require you to sign those agreements, I worked for one where they gave one to you after years of employment* and you had to sign if you wanted to continue to be employed. Lawsuits are spectacularly beside the point. You don't say to your spouse and kids "they fired me today, but I'm sure in ten years or so we'll win the lawsuit!"

*if this sounds implausible, it might have been at the time that a spinoff was happening and employees were being "technically" rehired by the new entity. I don't remember for sure.