Usually there is a little bit of both, depending on how much free time we have, what resources, equipment and space we have, and in what climate zone we live. For those of us who live in the great white north, some plants, such as tomatoes, peppers, watermelons, etc., must be launched inside in late winter or early spring if we want to eat them before August.

Although I have a small greenhouse where I can start seedlings, the cost of heating it in February or March will exceed the cost of buying a couple of dozen bedding plants, especially if you can find them at a good price. It makes sense to just buy litter plants launched in a commercial greenhouse and wait for the weather to improve to start seedlings in my own greenhouse.