So you can write the new functionalities in the new popular framework and keep the old one running. Why that's a better choice... well it depends on case by case.

However this is a bit like a reverse proxy that load balances many different web apps. You could have different applications written in different languages serving requests to the same url.

Furthermore I don't see anything here that load balancers haven't done since the '90s. Maybe I'm missing something but maybe that's why everybody is puzzled.