Many fruit trees, bushes and vines are not grown from seed but by grafting. One variety forms the root stock and another (sometimes more than one) forms the fruit bearing part of the tree. In this way, fruitwood from one good fruiting tree can be grafted onto many different rootstocks, thereby propagating the plant without seeds. A further advantage to this method is that certain rootstocks are resistant to certain soil born diseases so they are used in favour of the natural rootstock. There are many other aspects and advantages and you can see them at Scientific American's Ask-An-Expert website.