The result is a webpage that can do lightening-fast transitions client-side without breaking the back button.
The code also usually required some (often significant) refactoring of the code in question to make it portable between the front-end and back-end environments. In classic ASP, this usually meant ensuring front-end code moving to the back-end was isolated from any DOM interaction and didn't use any ES5(+) features, while back-end code moving to the front-end had to be isolated from the ASP objects (Application, Server, Session, etc.). It's all stuff that seems like common sense, but wasn't rigorously applied to JScript code which only had to work on IE5 (later 6, finally 7 before the project was put out to pasture), especially since use of JavaScript on the client wasn't significant until the project's last couple of years.