The 398 drawings are the reason the manuscript is used the way it is. They are pen and ink, full page, captioned, and they show things no text in the corpus describes: how a mummy bundle was carried, how a khipu was held, what a diviner did with a fire, the posture of a person addressing a huaca, the tools of a mine, the layout of a village church.
Historians of Andean religion who cannot recover a gesture from a Spanish paraphrase can sometimes see it here. The drawings also carry an argument of their own. Andean figures are shown with dignity and correct insignia, Spanish figures are frequently shown behaving badly, and the placement of figures on the page follows an Andean spatial logic that scholars have argued encodes rank and moiety. Reading them is not straightforward.
Guamán Poma had absorbed European print conventions and was drawing partly in that vocabulary, so a European sign and an Andean one can sit in the same image. He also had reason to present what he presents. But a picture made by an Andean hand in about 1615, of Andean practice, by someone who had watched extirpators work, is a category of evidence that scarcely exists anywhere else in colonial South America. Scholars now cite the drawings by folio number in the same way they cite a passage of text, which is a fair measure of the evidentiary standing the images have acquired in the last half century.