What is the use of Sign on URL when setting up SAML on Microsoft Entra?

I am not sure what is the use for the Sign on URL as seen here:

https://learn.microsoft.com/en-us/entra/identity/enterprise-apps/media/add-application-portal-setup-sso/saml-configuration.png

(Source: https://learn.microsoft.com/en-us/entra/identity/enterprise-apps/add-application-portal-setup-sso)

Edit: In the picture above, Sign on URL is marked as required, but in my case it says optional.

From what I understand, if I set this URL up, it will redirect the SAML response back to this URL instead of the ACS for further login on the SP side.

So if for example my app needs a login page after being authenticated by the IDP, it will redirect to a login page.

But why can't I just use the ACS to redirect to a login page?

The Sign-on URL in Microsoft Entra SAML SSO configuration is the entry point to your application for initiating the SAML login flow. It’s used when a user selects your app from the My Apps portal or any user-initiated flow from Microsoft Entra ID.

• The ACS URL is where Azure sends the SAML response after authentication. You can’t redirect to the ACS directly because it's only meant to receive assertions, not initiate logins. The Sign-on URL is optional if you're only using IdP-initiated SSO.

For sample, This URL does not point directly to the IdP login page (e.g., https://login.microsoftonline.com/...). Instead, it points to your app — for example, https://myapp.com/login.

• When Microsoft Entra ID redirects the user to this URL, your app is responsible for initiating the SAML authentication request by redirecting the user to the IdP (Azure AD), which handles the login.

I created a Microsoft Entra SAML Toolkit and configured SAML SSO like below:

• Reply URL (Assertion Consumer Service URL),: https://samltoolkit.azurewebsites.net/SAML/Consume.
• Sign on URL: https://samltoolkit.azurewebsites.net/

I tested the SSO app and logged in like below:

Logged in successfully which redirected me to Sign on URL which is https://samltoolkit.azurewebsites.net

When a user clicks your app in the My Apps portal, Microsoft Entra ID sends them to your app’s Sign-on URL, which initiates a SAML request and redirects them to Microsoft Entra ID for login. After authentication, Microsoft Entra ID sends the SAML response to your app’s ACS URL, where the assertion is processed and the user is signed in.