When planning my garden there are many factors that I consider in regards to the functions that can be obtained by the different types of plants and their intrinsic characteristics. Some plants have multiple functions. The more functions that a plant checks off, the more likely I am to plant it, abundantly. The first things that come to mind are:

Fertility - Ability to aid soil fertility, adding carbon, nitrogen or feeding microorganisms
Food - Does it provide food for me or forging animals?
Flowers - Make me smile and smell amazing. More smiles are a good thing.
Bees/Pollinators - Does it attract and feed the insects that are responsible for pollination?
Beneficial insects - Certain types of flowers (i.e…umbels - parsley, dill, fennel, cilantro) attract insects that eat other insects which we consider pests.

Companion planting is a practice based on observations and a little bit of science. Follow this link for a simple chart which shows companion as well as antagonists, or see at the bottom of this article.

Below are a few principles that I keep in mind when considering companion planting.

It is beneficial to plant things that add nitrogen to the soil.
It is beneficial to plant things that deter pests. i.e. garlic or society garlic.
Roses are good for planting in areas where there may be fungal issues. The rose exudes anti-anti-fungal chemicals from its roots. We want more fungus than bacteria around our fruit trees, so I keep the roses away from the fruit trees.

When we think about plants being next to one another it is common that we speak about the competition that takes place for resources between them. What I find least common is the conversation based around the cooperation taking place between certain plants which are sharing space. Generally speaking, there are seven different root structure patterns for plants. Plants of different forms can be spaced closely together if they are not all competing/filling the same niche. There are flat rooted plants, tap rooted plants, heart rooted plants, fibrous, etc. For example, the roots of densely spaced 350 foot tall, Coast Redwood, on the California coast only go 6 feet deep. They are all flat rooted. This is because they need a year-round supply of moisture which is given by the condensation of fog in the summer months. The roots of these trees interlocked with other specimens to share the niche of water and nutrients. This allows for shade tolerant plants with different root patterns to take up water from the deeper soil. I find this to be a great example of cooperation, not competition.