Earlier editions of the Springer Graduate Texts in Mathematics series include a motivating statement about the purpose of the series. The current statement of purpose is as follows:
Graduate Texts in Mathematics bridge the gap between passive study and creative understanding, offering graduate-level introductions to advanced topics in mathematics. The volumes are carefully written as teaching aids and highlight characteristic features of the theory. Although these books are frequently used as textbooks in graduate courses, they are also suitable for individual study.
I believe the earlier statement of purpose, from the paper books, is somewhat more colorful. Does someone have access to an early GTM book to check? I believe the statement is found just inside the cover; it isn't present in digitized versions that I've seen online.