Especially the responsive modifiers are a huge enabler. We are actually several month in the making of a very specialized mobile website which is gonna be released as an android and a ios app. Recently we are asked to add a desktop version. I was really concerned about our given deadline and said to the client, it would be possible but would probably take one or two weeks to develop. After telling my colleague that we have to make an desktop version out of the app, we were both concerned. But it turns out that after one day in the making, we did have a fairly good looking desktop version which the client really liked.

I am pretty sure that with plain old css and our poor architecture we wouldnt be able to deliver in time.