I don't think it makes any sense to define
roles anywhere else than in
admin database. If you define them somewhere else then likely your user/role management get unmanageable.
Except for roles created in the admin database, a role can only
include privileges that apply to its database and can only inherit
from other roles in its database.
A role created in the admin database can include privileges that apply
to the admin database, other databases or to the cluster resource, and
can inherit from roles in other databases as well as the admin
So, creating roles in "non-admin" database is much more limited. However, creating users and roles is usually an admin task and typically admins have full power anyway. Perhaps when you like to provide a MongoDB playground (e.g. https://mongoplayground.net/) then such limits could be useful.
Note, when you run a MongoDB Sharded Cluster then users and roles are commonly defined in the Config Server Replica Set. When you connect to a Shard directly then the shard contacts the Config Server in order to authenticate your connection. However, when your shard is disconnected from the cluster then it cannot contact the Config Server. For this you define a Shard Local User directly on the shards, see Create the shard-local user administrator. I think this is the only useful exception where users are not defined in common