I see grafting mentioned a lot for apple trees and i see some are grafting other types of trees aswell. What's the benefit of this? Is this for cross pollination reasons? I'm wondering if i should be doing this with my apple trees?
Growing many fruit trees from seed does not mean you will get the fruit of the seed it came from. For instance, an apple tree grown from a Honeycrisp seed is not likely to produce Honeycrisp fruit. In order to replicate the fruit exactly you have to graft budwood or scions from the variety you want to produce onto rootstock from any variety grown from seed. If you bought your apple trees from a reputable producer and they were a named variety then it is likely they were already grafted when you bought them. But if you seed your own you will need to graft.
A perfect example if this are avocados. Many people sprout seeds. Some of those sprouted trees will eventually produce fruit subject to a few factors (tree sex, type, etc.). Most of the time that fruit isn't edible. Very rarely an edible variety is discovered. That's where the Hass avocado we all buy in the stores came from (California in the 1920's). And every Hass since then is a graft from that original tree (or graft of a grafted tree, etc.) that was seeded.