Plaque accumulation can cause two major problems: gingivities (gum inflammation) and periodontitis (bone inflammation).
In absence of flossing and brushing it takes about 24 hours for the first stage of plaque formation. That is why it is recommended to brush and floss at least twice per day.
Even after brushing some bacteria can get under the gum and start to build plaque. In the abscent of proper care calculus will be produced later which causes harm even to the bone supporting teeth.
These bacteria are also one of the reasons for bad breath. By eliminating them we get a fresh breath and healtier gum.