when should you think about getting a federated identity provider

hey everyone, i'm seeing more and more apps use federated identity solutions. it seems like a good way to manage user sign-ins across different services, especially now with single sign-on becoming so popular. what do you think are the main signs or moments when it's absolutely necessary to get a federated identity provider (think Okta, Auth0, or even building your own with Keycloak) instead of just wishing you had one? when did you switch over, and what were the biggest upsides or difficulties?