If you take a seed from an apple tree (for example, a Honeycrisp apple) and plant it into the ground, you will indeed get an apple tree — but it won’t be a HONEYCRISP apple variety. It will be something else, the product of the parent tree/pollinator tree. Basically, a child of the parents, which certainly isn’t the same as the parents….
BUT, if you graft onto a different rootstock, you can do two things…
1. Control how big the tree will get by controlling the rootstock (dwarf, semi-dwarf, standard) and
2. Get the desired traits (Honeycrisp) since you cut a honeycrisp limb from the “parent” tree and graft it onto the rootstock.
You propagate the original DNA basically, and you aren’t getting interference from Mr. Mendel and genetics.