There always comes a time when you need to thin out your plants.
ALWAYS first find the strongest healthest plant in a group and thin those crowding that plant.
There are diffirent way to do this for diffirent plants.
The most common way is to lop off the top of the plant at ground level. This method is used so you do not disturb the roots of the plant you plan to keep which can upset and sometimes kill the plant. This works for most plants but root plants.
For thinning root plants (carrots, turnips, beets, etc) just lightly pull the baby plants out of the ground and toss into the compost.
Thinning is important in all plants.. if you don't thin let's say carrots you will have small short disfigured carrots now let's say you didn't thin your tomato plants... you have 3 plants where one should be resulting in small dissapointing yields and very unhappy plants as their roots are overly crowded
Happy Spring Gardening!