Demotic was the ordinary written Egyptian of the country for roughly a thousand years, through the Late Period, the Persian occupations, the Ptolemies and into Roman rule. Its decline is documented with unusual precision. Greek displaced it in administration after the Roman conquest, its use narrowed to temple contexts, and the latest known Demotic inscription was cut at Philae in the middle of the fifth century of the common era, within a few decades of the last hieroglyphic one at the same temple.
What replaced it was Coptic, the same language written in Greek letters with a handful of signs borrowed from Demotic for sounds Greek did not have. That handover is the reason Egyptian survives in liturgical use today: a Christian community adopted the alphabet of the empire and kept the language underneath it.
For the atlas, Demotic matters as a religious source in two directions at once. It preserves temple ritual, priestly instruction and magical practice from the period when Egyptian religion was under pressure and was defending itself, and it records the ordinary transactions of the temples as institutions, their leases, their endowments and their disputes. Very little else lets a reader watch a religion working as an economy and as a devotion within the same archive.