The identity provider login URL in the usage tab, if used as is, allows to start a SAML IdP-Initiated flow where a SAML assertion will be sent to the configured callback URL.
In addition to IdP-Initiated flow it would also be possible to start a SP-Initiated flow by accessing that URL and including a SAML request as a query string parameter. This would also result in a SAML assertion being sent to the configured callback URL.
You should clarify what do you mean by having the app setup with it’s normal authentication. It’s important to have in mind that the same client identifier of the SAML addon application may be used to start an OIDC/OAuth 2.0 connection at the
/authorize endpoint and this would indeed NOT send a SAML assertion to the callback URL, but this would mean there’s some expectations incorrectly set somewhere.