Generating value is only a small part of it. You can get fired or laid off for any reason in most places in the US. You can be doing great work personally, but if upper management screws up, or the business environment is just bad (despite management's best efforts), you'll get laid off as soon as it makes financial sense for the company to do so. You can also get canned or at least mistreated to the point of misery because of interpersonal problems having nothing to do with your performance, a terrible manager, etc.