Seed sowing is great fun, and if you have not tried it then you should. However, not all of us have the time or facilities. Germinating seeds takes space and, if you have to buy all the accessories, you won't save much money overall, so buying "ready-made" seedlings is a sensible way to go for a lot of people. You get the right number of the best quality plants, delivered to you at exactly the right time to plant.
Please note that annual plants are not covered by our year guarantee, because they are not supposed to last that long!