Most tomato varieties belong to the cordon or stake tomato plants which means that their main shoot will continue growing in height if it is not clipped at some point. This is why they need to be protected and supported by 'a stake' or grid, which is where their name originates from.
Due to their tendency to grow up to impressive heights it is recommended to prune and cut out their lateral shoots regularly and to keep them as single shoot plants. If kept untrimmed, these tomato plants would develop plenty of very dense foliage, keeping sunlight from reaching the fruit and increasing the storage of moisture. Hence, the tomatoes would ripen with difficulty and the plants would be more prone to the dreaded diseases such as leaf blight and brown rot.