One of the places missed by brushing alone is directly in between the teeth. And if there is no disruption of the bacteria in between the teeth a cavity forms.
Flossing the one of the best ways to disrupt the bacteria in between the teeth that brushing alone won't reach. One of the worst things you can do is leaving debris on your teeth during the sleeping hours which if becomes a habit will causes dental problems.
Use floss at least once daily and especially before bedtime.