I don't recall any reason to do this other than using the same user and auth key to authenticate in different Ceph clusters, like in a multi-backend solution, or just because things get messy when you are not using a default configuration.
Sometimes, things gets easy when we use the same user and auth key on both clusters for services to connect to, so lets see some background commands for managing users, keys and permissions:
Create new user and auth token (cinder client example):