If you don't thin crowded young plants, your crops may be stunted, poorly developed or not bear produce at all.
Most seed packets indicate the proper spacing between plants.
Follow seed packet or garden book instructions closely. Don't wait too long to thin. Root crops, including onions, carrots and beets need space to grow both above and beneath the ground surface. The leaves of these plants are essential for gathering the sunlight the plants need to manufacture the carbohydrates stored in the edible root or bulb. The distance between the plants is the size the root will be, so give each plant plenty of space.