I'll edit for three main reasons.
Dietary balance. I like to keep a diverse diet, balanced between different food groups. If the output of the garden is tending too much to fruit, or leaves, or whatever, I'll reduce that set of plants to make more space for something I'd like more of.
Concluded experiments. I don't like to say 'failed' experiments, but having tested some plants it's time for them to come out or be relocated. Some over-aggressive species end up in pots. Or the plant might not thrive, or I might not like it. Temperate forest
gardening is still very much an experimental adventure, and you need to test things in your own garden and kitchen. I am fairly slow to completely get rid of a plant, as a new growing or cooking technique has often turned a disappointing plant into a stalwart after many years of experiments. To an extent I let the garden self-edit. When I get a new plant I'll usually plant it in a few different spots in the garden. Usually I'll then thin (or the plants will self-thin) to the place where it does best - although with some 'spready' pants there is an argument for thinning them to where they do worst!
Density. If self-seeded plants are coming up too thickly or it turns out I've planted something too close, I'll thin. Overcrowded plants are usually less productive of the parts you want than well-spaced ones.