Healthy plants balance their foliage and root growth. Good pruning and repotting habits help plants to maintain this balance.
A rootbound plant will grow foliage energetically until the roots are unable to feed the entire plant. Foliage growth will stop until the plant is repotted into a larger container. Foliage will grow again only after the roots have a chance to catch-up.
Pruning either foliage or roots affects this balance. Regular foliage pruning or pinching back encourages fuller growth and also provides cuttings with which to propagate new plants.