"The Coptic New Testament is among the primary resources for the history of the New Testament text. Important as the Latin and Syriac versions may be, it is of far greater importance to know precisely how the text developed in Egypt. The Alexandrian and Egyptian text types are not only of the greatest importance by far, but the special climatic conditions of Egypt have also preserved for us nearly 100 percent of all the known witnesses to the New Testament text from the period up to the fourth century." –– Kurt and Barbara Aland, The Text of the New Testament (Eerdmans, 1987), page 200
Modern translators know the value of having a good text from which to translate. The New Testament versions found in newer Bibles are based on Greek texts that have been constructed on the basis of the earliest known Greek manuscripts available, as well as insights from early translations into other languages, such as Latin, Syriac, and Coptic.
The Coptic text also indicates that its translators were skilled textual critics. They had variant readings from several different Greek texts before them, and some of these they incorporated into their master text. There are readings in the Coptic text that give evidence of knowledge of readings presently preserved in ancient Greek manuscripts such as p66 (P. Bodmer II, 2nd century), Sinaiticus (4th century), Alexandrinus (5th century), Ephraemi (5th century) and Bezae (5th century), among others. These are considered to be among the most accurate of the early witnesses to the New Testament text.
Consider, for example, John chapter 8. At verse 38, many translations follow a Greek text that read: A EGW EWRAKA PARA TW PATRI LALW KAI hUMEIS OUN A HKOUSATE PARA TOU PATROS POIETE. However, the Coptic text follows a Greek text that read: A EGW EWRAKA PARA TW PATRI LALW KAI hUMEIS OUN A EWRAKATE PARA TOU PATROS POIETE. The Coptic text follows the reading of the earliest extant witness to John's Gospel, that found in p66.
There are many such gems of early textual criticism to be found in the Coptic text of John, making it a fascinating subject for further research.