Bad habits interrupt your life and prevent you from accomplishing your goals. They jeopardize your health both mentally and physically. And they waste your time and energy.
Most of the time, bad habits are simply a way of dealing with stress and boredom. Everything from biting your nails to overspending on a shopping spree to drinking every weekend to wasting time on the internet can be a simple response to stress and boredom, but it doesn't have to be that way. You can teach yourself new and healthy ways to deal with stress and boredom, which you can then substitute in place of your bad habits.