For three thousand years Egyptian was written in scripts that recorded consonants. Hieroglyphs, the cursive hieratic derived from them and the highly abbreviated Demotic of the later periods all worked this way, supplemented by signs that classified a word rather than spelled it. A reader who already knew the language supplied the vowels; a reader who did not was stuck, which is why the pronunciation of ancient Egyptian is conventionally supplied by Egyptologists inserting a neutral vowel between consonants, a convention of the discipline rather than a reconstruction.
Coptic ends that silence. Because the Greek alphabet has vowel letters and Coptic took them all, including both the short ei and the long eta, a Coptic word shows its vowels. This is why Coptic is treated as the key to the vocalisation of earlier Egyptian, and why arguments about how a New Kingdom word sounded so often end up citing its Coptic descendant. There is a limit to how far that evidence reaches.
Coptic is separated from Middle Egyptian by something like two thousand years of sound change, so a Coptic form is a late witness rather than a direct one, and the dialects disagree with each other in exactly the vowels at issue. Eta is a reliable witness to Coptic and a suggestive one to what came before, and the distinction between those two things is the whole methodological problem of the field in miniature.