Should web sites that use windows authentication require users to login to the web site?

Question

This may be a dumb question, but why would an internal corporate web app that uses Windows Authentication ask for my Windows credentials before authorizing me to use the functionality in the web app? I would think that if it were using Windows Authentication that the web app should be able to determine my identity based on how I logged onto the domain and authorize me accordingly.

The only reason that I can think of why a website would require me to log into the site using my Windows credentials is if the site really uses forms authentication, for example, and verifies your credentials against a table that is fed from the Active Directory.

Should web sites that use windows authentication require users to login to the web site? It seems like this is an unneccesary step.