After all seven Devices are restored, players will get an Achievement worth five Primogems.

Source: https://gamerant.com/genshin-impact-chess-piece-location-temple-of-gurabad-safhe-shatranj/

One of the main reasons why the overall sales seem so low is the fact that a lot of fans are saving up their Primogems in order to pull for one of the upcoming Fontaine characters.

Source: https://gamerant.com/genshin-impact-eula-klee-banner-revenue/

The first-time completion rewards for the City of the Deceased Domain include 40 Primogems, five Dendro Sigils, one 4-star Desert Pavilion Chronicle Artifact piece, two Guides to Praxis, three Hero’s Wits, five Enhancement Ores, and 45,000 Mora.

Source: https://screenrant.com/genshin-impact-unlock-domain-city-of-deceased-location/