If you want to start growing and you do not have weed plants yet, then you start buying weed seeds. You could also buy cuttings or clones from somebody else and continue growing with these. However, cloning has a couple of disadvantages. First of all the clones are not sterile and with cloning you also bring along the insects and fungi from the growing room of the other person.
The big advantage of cloning is that you know for sure you are dealing with females. However with the emergence of the feminized or female seeds, there are ever less growers working with seeds. Germinating is easy and you are able to time perfectly when you want to start and when you want to harvest.