Tomato ‘laterals’ or side shoots, are the stems that appear in between the stem and a branch. If left to grow, they will become another main stem with branches, flowers, fruit and more sideshoots of their own.
‘Pinching Out’ or removing the sideshoots is recommended because if they are left to grow, the new stem will compete for nutrients with the original plant and the fruit.
If you leave the sideshoots to grow, the tomatoes might be smaller and could be slower to ripen. Pruning tomato sideshoots is really just like thinning the plants.
However you don’t have to prune out sideshoots in every variety of tomato.
Pinching 0ut tomatoes is not essential and some gardeners don’t bother with tomato pruning at all. However, most agree that you will get a better yield from tall tomato plants if you do.
It does also depend on the type of tomato plant you are growing.
As a rule of thumb, vine or cordon tomatoes (also known as indeterminate) will need pinching out whilst bush or tumbler tomatoes (also known as determinate) do not.
Indeterminate tomato plants can get extremely large and will keep producing tomatoes all season, so they can handle the pruning. If you leave all the suckers to grow, the plant will favour the growth of more stems rather than ripening the fruit. Regularly pinching out of tomato side shoots will concentrate the plant’s energy into producing fruit.
This is a photo of the Gardener’s Delight variety I am growing in a window. You can see the side shoot has become bigger than the actual stem, it should have been removed.
As long as you have a strong main stem, you can leave a few sideshoots on the plant.
Some gardeners only pinch out everything below the first flower cluster, while others leave a couple of sideshoots on the lower portion of the plant.
I am told that tomato pruning is more trial and error than precision, I am still learning.