Why use the yield keyword, when I could just use an ordinary IEnumerable?

Answer 1

With the "list" code, you have to process the full list before you can pass it on to the next step. The "yield" version passes the processed item immediately to the next step. If that "next step" contains a ".Take(10)" then the "yield" version will only process the first 10 items and forget about the rest. The "list" code would have processed everything.

This means that you see the most difference when you need to do a lot of processing and/or have long lists of items to process.