A reality check is like going to the Doctor for a check-up. It's uncomfortable being probed. It's anxiety-provoking waiting for test results, but at least you know what you're dealing with. If there is bad news, you are generally in better shape finding out before a potential problem gets aggravated.
You can get a reality check anytime you test your assumptions about what is going on around you. For example, a business needs information about what their customer really wants. At work, you will need some feedback from those people who will have the greatest impact on your success. At home, you need to know how the people who are closest to you are feeling about you.
Getting frequent reality checks helps you cut through your own illusions and ultimately be much better equipped to deal with what is really going on. You get the chance to see if you're distorting reality in a way that could leave you vulnerable to unnecessary damage.