Repelling Pests & Attracting Help
Some plants emit chemicals from their roots or leaves, called allelochemicals, which repel pests. As an example, tomatoes repel caterpillars from diamondback moths, which like to use cabbage leaves for food.

Other plants attract insects that prey on pests that would otherwise damage nearby plants. As an example, beans attract insects that eat corn pests, such as leaf beetles.

Space And Other Factors
Plants that need partial shade often grow best in the shade of a larger plant or bush. As an example, spider flowers (cleome) can provide the partial shade that lettuce prefers. Sometimes a row of sturdy plants can protect weaker plants from wind damage.

Root depths vary from one plant to another. You can take advantage of this difference to grow more vegetables in a given area. As an example, by planting shallow-rooted onions in close proximity to deep-rooted carrots, you can grow more of each in your vegetable garden.

The Unexplained
When basil is planted in close proximity to tomatoes, both grow very well. This is a beneficial relationship that hasn't been explained. Another similar relationship is between climbing beans, corn, and squash. When the three grow together, they are all happy, but know one knows exactly why.