Like Single Sign-On (SSO), and for stuff like SSO-only homeservers. Probably by implementing the User-Interactive Authentication API
This may become more relevant in the future where logging into Matrix is not username/password and is instead OIDC (or some other SSO), making synadm matrix login non-functional because it is only username/password.
Resources:
Considerations when implementing:
Note: I don't know if the OIDC thing is SSO. If that's not the case, this issue may have to split into SSO and future OIDC logins