The company claiming something you said, even out of context, could be interpreted as coming from the company. If you choose to disclose you work for a company, you become a spokesperson for that company unless you disclaim those words (even then, there are other considerations to make regardless of whose opinion is being expressed, because you linked yourself to the company.).

By putting that, they decrease the likelihood of reprocussion in the workplace for things said outside of the workplace.

You can still get in hot water for anything you say that ties back to you or the company regardless if you disclose who your employer is.

This is the grey-area that corporations typically carve out in a social-media policy so that employees can engage in discussions around their employer without being on behalf of their employer.

It's still a perilous position to put yourself in as an employee. Innocent and innocuous things can always be misunderstood or misinterpreted.

What happens when you use that disclaimer and are self-employed though?