They're not sterile, but with a wild apple or grape seed there's no guarantee as to what it will produce. Lots and lots and lots and lots of breeding went into the fruits/vegs we have today. When a good plant does come along then the only way to grow additional plants is by cloning it -- taking a cutting from one of its branches.
The purpose of grafting as pertains to apples is to control the size of the tree (standard, semi-dwarf, dwarf etc.), the freeze resistance, flood/drought tolerance, strength, etc. As pertains to grapes I believe it is primarily for disease resistance.
The purpose of grafting as pertains to apples is to control the size of the tree (standard, semi-dwarf, dwarf etc.), the freeze resistance, flood/drought tolerance, strength, etc. As pertains to grapes I believe it is primarily for disease resistance.