CarlManvers2019
Writers Blocked Douchebag
What does he do with it after pricking himself with said arrow and getting a STAND POWER
jojo.fandom.com
He realises it can be used to give other people STAND POWER as well
Presume this is after he becomes a signatory of the Unseelie Accords

Bow and Arrow
The Bow and Arrow (弓と矢, Yumi to Ya) (more specifically the Arrow (矢, Ya)) is a powerful item introduced early in Diamond is Unbreakable. In Part 4, it becomes a major plot-driving force for its role as the source of many new Stand users; and it is revealed that its influence extends back to...
He realises it can be used to give other people STAND POWER as well
Presume this is after he becomes a signatory of the Unseelie Accords