Why did Allah Almighty describe the books of the the People of the Book as confirming what they had, despite their distortion?

The Holy Quran came to confirm what the previous heavenly books brought forth—before their distortion—regarding monotheism, virtues, and admonitions. Thus, it encompasses the guidance that the prophets called for. Any differences between the Quran and previous scriptures are due to varying interests and eras, and this is considered abrogation, not invalidation. To deny the Quran leads to denying what came in previous books, because what it brought is what the prophets before it brought, and it contains the description of Prophet Muhammad and the glad tidings of his coming.