Why is it so important not to leave a cat alone a lot?

All of our cats are fully domesticated and need human interaction. Leaving a pet cat unattended for 10 hours or more will drive them into behavioural issues out of boredom. 

These behaviors include biting, destroying furniture and household items, urinating outside the litterbox to show distress or depression, loss of appetite and health issues. 

Also, an illness may go unnoticed. Finally, accidents tend to increase when cats are alone often.