Event loop + callbacks allows you to fully utilize any given thread (meaning that while there's outstanding I/O or other long-running operation, the thread isn't idling waiting for it to complete, but instead performing some useful work). This is completely orthogonal to multithreading - if you have multiple threads, there's still benefit in fully utilizing them, and so each thread gets event loop and callbacks. It's true that some popular frameworks in this vein (notably, Node.js) combines event loop with single thread per process, but it's not a prerequisite. In C#, async/await is commonly used with multiple threads, for example in ASP.NET web apps.