is to provide food for the beneficial microorganisms that release nutrients into the soil as they decompose the organic matter. Earthworms and other soil dwelling insects aerate the soil as they move through it and contribute still more organic matter with their waste and decomposition. This makes for what is called. Fortunately we don't need to know everything about soil to grow our plants and in most container gardening you won't be using garden soil. When you remove topsoil and put it into a container you lose many of those natural support systems and are left with material that is too dense and heavy for plants to grow in well.
So for container (planter) gardening you try to simulate the best conditions topsoil can offer, adequate air, beneficial microorganisms, organic matter and nutrients. Trying to do this in a form that won't compact and be too dense.