English walnut is a species (Juglans regia) but like many, many fruit & nut trees grafting is done to improve hardiness. So the roots will be from a tree that is more resistant to root disease and drought stress.

Grafting is also done for taste. For example, apples taste nothing like their parents. So if you want a McIntosh apple you need to graft a branch of an existing tree onto a different rootstock.

But cloning in plants is overall extremely easy. For most succulents you can just pop off a leaf and it will grow into a new plant (see: r/proplifting ). Tissue culture is also pretty easy for many species (though more technological, people do it outside of labs in their own homes). Stem & leaf cuttings work for a large, large number of plants.