If you're relying solely on brushing to keep your teeth clean, you're missing nearly half the surface area of your teeth which, not surprisingly, lies between them. For that reason alone, flossing should be an essential part of your oral care routine and never an optional extra.
By using floss to remove the plaque from between your teeth, you're helping to prevent gum disease, tooth decay, and halitosis (otherwise known as "bad breath"), a considerable amount of upside for just a couple of minutes effort each day.
A part of your routine
It's always best to floss when you're not in a rush or when you're too tired to do it well. If you find you're exhausted at the end of the day, then it's a good idea to floss first thing in the morning or after lunch. Alternatively, if you like to go to bed with a clean mouth then floss before your nightly brush. If you have kids, they should begin flossing, with your help up until about age 8, as soon as they have two teeth in contact.