To give an example: I'm working on something that is on one level a simple web app. But then, I needed to code what amounted to a concurrent merge sort (merging multiple streams of information (and their histories) that are each ordered but not relative to each other — think Twitter streams, since that's what they were.)

Because I have a degree in computer science, that theory background meant I could quickly recognise the type of problem I was trying to solve and apply standard stuff I had learned to it. Without that theoretical background, I would have developed something that worked, but it would have taken me longer, it wouldn't work so well, and would probably break in unpredictable ways :)

In short, its never just web development.