When combining two plants in one pot we can create a system where, if we choose the right plants, they benefit each other! I also find that mixed pots just look better.
Some plants need a bigger plant to protect and shelter them.
Shelter
There are many places where having two or more plants in a pot can help various plants meet their needs for protection from harsh sunlight, maintenance of a microclimate, and root health.
I once visited a guy in Slovakia who had a garden on his balcony. He had potted maize plants with runner beans growing on them.
Productivity
In nature, we often find that if a catastrophe happens and a piece of land ends up clear of vegetation that a succession of plants will move in and recover the soil.