The OS uses a minimal amount of cpu ny itself that it doesn't really matter if it's dual core or not. However, when you arrive at a javascript heavy website and switch away to do something else, you can do that other thing at full speed while the javascript driven website continues loading at full speed in the background, for example.

Depending on the web browser, you can have one tab loading on the cpu while rendering the other and viewing it on the other cpu. It doubles your available cpu power...