Like some times you just feel bad, or for mental health need to take a day. But you have to call in and pretend to be sick, or use designated time off that you've accrued, and/or some jobs require you to personally find someone to cover your shift. If your presence on any random day is integral to the functioning of the business then that is a shittily designed business. “I just don't feel like it today” should be a valid reason for taking a day off.