In Diablo 3, some people like to use a stone of jordan for certain specific purposes. Tempest Rush monks like them as they are a good source of additional spirit regeneration. Sometimes people will use these rings to help reduce the cool down timer on a certain skill, or to improve the damage of a skill.
Another somewhat common use is to switch over to an SOJ for elite fights. If you already have a very large amount of DPS output, it can be beneficial to take a sheet DPS hit in exchange for up to 30% extra damage against elites while you're fighting elites.
Check out the video to learn a bit more about the Stone of Jordan.