Why is it necessary to graft a tree to get the desired species and not rely on the tree's seeds?
There are few reasons. The two most important:
We selected varieties for size, durability, and taste (mainly about sweetness). Natural selection is long. We do not like to wait, so we have not yet stabilized the population: the genes will contain preferred and non-preferred traits. We seed the non-preferred trait could show up again (and often it does).
We choose graft to easy cultivation of plants: we select graft which can live on dry environment, on limestone, and now also often: to limit growth, so that plants are smaller: we plants more dense orchids, but it will be much easier to harvest and handle small trees.
So we use graft because we want advantages on both side.
A small note: sometime trees are not fertile: multiploid plants often produce larger fruits (multiploid means that the plant has extra copies of genes, so there is more production of proteins and living stuffs.). Often this make plants not or less fertile. This is also a plus: no or small (and fewer) seeds. Banana has no seed (Cavendish variety: the sweet banana).
Because in many cases, the tree does not come true from seed of its own fruit. If you grow, say, an apple from an apple seed, there's no way of knowing what the fruits will be like, nor the eventual size of the tree - it might be enormous, and with very tiny, sour fruits. The way growers get round that is to use propagation techniques to graft the desired fruiting parts onto a different rootstock - this is often done because the rootstock has a dwarfing effect, giving a smaller tree overall, or sometimes because the rootstock is more vigorous, meaning the grafted part is more vigorous and grows better. Apple trees, for example, come with a choice of rootstock; dwarfing, semi dwarfing, semi vigorous and vigorous. Dwarfing rootstock will give an eventual height of around 2.5 metres, whereas vigorous will mean a tree of 6-7 metres in height - but the fruit will be same because they're grafted.