Maybe back when standarts where on shaky ground and different versions of the same content was made? I too cant see the performance advantage of it. Deprioritizing less mainstream browsers to mess with the nerds?

Ahhh yes I remember those days... if you wanted to use advanced IE-only features, send to one codebase, if you wanted broader compatability, send to another. Similar to how mobile websites used to work. Thanks for the ideas! Any other hypotheses?