(A) It marks off the beginning mathematics from what went before.
(B) Ever since this discovery, abstraction has been a major theme in the development of mathematics, as those interested in the field have come up with ideas further and further divorced from their basis in the real world, and then sought ways to bring them back to tell us things about the real world which we might otherwise not have known.
(C) The discoverer of abstraction was the person who first realized that numbers are independent of the objects being counted, that two oranges and two apples (for instance) share a property, ‘twoness’, which is independent of what kinds of fruit they are.
(D) Abstraction, the action of divorcing properties of physical objects from the objects themselves, is a fundamental concept, perhaps the most fundamental concept, in mathematics.