Prune for plant structure and health
Suckers form in the axils between the leaves and the main stem. Encourage a strong main stem by removing all suckers below the first flower cluster.
With tomatoes, we want to maximize the efficiency of photosynthesis and minimize the risk of disease. This is best accomplished by ensuring that each leaf has plenty of room and is supported up off the ground. When a tomato plant lies on the ground, or when its growth is extremely dense, many of its leaves are forced into permanent shade, greatly reducing the amount of sugar they produce. If a leaf uses more sugar than it makes, eventually it will yellow and drop off. A pruned and staked plant will produce larger fruit two to three weeks earlier than a prostrate one.
A properly pruned and supported single-stem tomato plant presents all of its leaves to the sun. Most of the sugar produced is directed to the developing fruit, since the only competition is a single growing tip. The result is large fruits that are steadily produced until frost. If more stems are allowed to develop, some of the precious sugar production is diverted from fruit to multiple growing tips. Fruit production, although slowed, never stops. The result is a nearly continuous supply of fruits throughout the season. In general, more stems means more but smaller fruits, which are produced increasingly later in the season. (This is much less applicable to determinate plants, due to their shortened growing season and better-defined fruiting period. Therefore, determinate plants require little pruning. See "Indeterminate vs. Determinate," below.)