Trees are often grafted onto rootstock for several reasons. They include:
Often trees that have been grafted, like many fruit trees and some ornamental trees like cherry, will sprout from the rootstock. This is the area below the graft or bud union. This should be removed as soon as it grows so valuable energy is not drawn away from the growth you do want.
Use clean sharp secateurs and cut them off as deep as you can if the growth is from the ground or leave a 5mm stub if the growth is from the trunk.
You may need to do this several times over a year and it’s best to keep on top of it or you might find the suckers take over the desired grafted variety. This is because the rootstock is always superior to the grafted variety which is why it has been used.
If you have suckers popping up on your lawn, they will be from rootstock so if you love the tree and want more of them, you can easily pull them out, they may have roots attached and you can plant them and increase your specimen.