When goods are bought and sold, the buyer expects to become the owner of the goods, and to have the exclusive right to possess and use them. They do not want some other person come along with competing claims to ownership and possession.
To give buyers sufficient assurance that their rights of ownership and possession won't be disturbed, the law requires a seller to guarantee that the buyer will become the owner of the goods bought, and also that the buyer's possession of those goods will not be disturbed.