Why was this made and how was it used?
Relics such as this arm bone sliver needed reliquaries to keep them safe and honor their significance to the patrons or pilgrims that came to see it. Reliquaries like this priest were often made for a specific church or clergyman to be used in masses or on feast days. The small statue would have been either hung from the ceiling or placed on a tall pedestal to make it visible throughout the church while also securing it in a place that thieves would have a hard time accessing. The gold sheen that originally covered the statue would have glittered in the sun or candlelight to give a divine aura to 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 was most likely loose in its original resting place in Egypt, brought back there by Pope Michael V, so it needed to be made a reliquary once it got to France. The composition of the statue draws focus to the relic and its fine 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.