Why was this made and how was it used?
Relics such as this sliver of a saint's arm bone needed reliquaries to keep them safe and to honor their significance to owners or pilgrims. Reliquaries like this priest were often made for a specific church to be used in masses or on feast days. The gold color that originally covered the statue would have glittered in the sun or candlelight, providing a divine aura for the relic.
This priest appears to have specifically cast or repurposed for this arm bone relic. If relics were taken from a previous collection, they were often kept in their original reliquaries, as the exotic quality of the art's craftsmanship raised its value in its new homeland. The bone of St. Macarius was most likely loose in its original resting place in Egypt, so it needed to be made a reliquary once it arrived in France. The composition of the statue draws focus to the relic and its fine fabric wrappings, and the enameling that was originally present on the relic tube would have made its bright colors stand out even more against the metallic surface of the rest of the statue.