It is because this ground or sugatagarbha or potential is common to all beings that they are capable of attaining enlightenment. If they did not have such a ground then they could never become buddhas.
For example: a stone doesn’t have the ‘ground’ or potential to produce oil, and so no matter how much you might press it and grind it—even if you use modern tools and machinery—you will never extract any oil. A sesame seed, on the other hand, does have the potential to produce oil, and by pressing it in the right way, sesame oil can and will be produced. So it is because the potential exists as part of our basic nature that we can become buddhas.