Removing the suckers, called suckering, encourages earlier, larger tomatoes, and it keeps the number of fruits low in relation to foliage, which improves the flavor of the fruits.
Suckering is the process of removing the side shoots that grow from the node where a leaf attaches to the tomato vine. There are many different techniques. One method is to allow three suckers to develop into strong stems. Then keep pinching new suckers that form to encourage the plant to put its energy into fruit production.