But, it is a horrible WTF, indeed.
I think the WTF is the convoluted route that animated GIF took to get to the desktop:
Pretty sure you could also just click
cancel as this wasnt a logon for the pc, it was network credentials -
I recall that, also.
You could then specify credentials when attempting to access network resources (shared drives, printers, etc) and also browse to someone's (local) profile to view their content.