If these are not removed, each sucker will become a main stem with flowers, fruit and even more suckers. By pruning tomatoes, you are reducing some of the potential fruit the plant could produce. Rather than a plant producing 80 fruit, a pruned plant may produce only 40. Since there are fewer fruit each will weigh more. This doesn't mean that a cherry sized fruit will turn into a giant beefsteak type but it will typically add an ounce or two, depending on the level of pruning.
Determinate types are relatively small, often referred to as bushy or compact.
Each short branch ends in a flower cluster and the plant does most of its growing before any fruit are set. The tomatoes tend to ripen at once, usually over a short 2 to 3 week period. After most of the tomatoes are harvested the plant yellows and additional production is limited. If grown upright, the plants are seldom more than 2 - 3 feet tall. Some examples of this type include Pik-Red, Early Girl, and Super Bush.