Hello @jaevin9035 - welcome! Medplum supports both our own authentication solution and bringing in external IdPs - which you noted in your post, and hopefully I can shed some light on the use cases.
In many medical apps and health record systems there are four major groups of users, each of which may have good reason to have their own IdP. They are :
Practitioners, Referring Providers or partners, Patients and Applications.
1. Medplum IdP is useful in that it is built in and works right away. It's also an implementation of SMART-on-FHIR authentication, which is a requirement for certified EHRs. It also has robust support for Client Applications which are often needed for integration jwt authentication and basic auth which have their own use cases.
2. Using an external IdP means that that IDP manages the password policies, 2FA and the like, Medplum counts on the external IDP to do that when it is used.
3. External IdP is very useful if you have other applications needed by staff and want SSO, for patient identities if you want to segment them, and to enable access by partners - so there are a lot of use cases. It can be useful for compliance reasons as well.
If you use Medplum, you can progressively support adding multiple IdPs for different groups.