Clearly, KRunner exists so that if Plasma crashes, I can run plasmashell --replace in there. 🙃
Alright, a somewhat more serious response: It gives people flexibility in how they can use their system. For example, the Application Launcher requires having widget for it on your panel. It requires having a panel.
Personally, I also just like KRunner, because it doesn't take me out of my workflow so much. The Application Launcher feels like I'm now doing the separate task of launching an application, with how it slaps that flyout into your face, covering your windows underneath. KRunner on the other hand feels like I'm merely adding another window.
In general, though, you might not find massively convincing reasons to have it, because why not? They use the same backend code. The tiny bit of UI to display KRunner, that doesn't cause much maintenance. Even if it did cause much maintenance, it's not like the volunteers working on that are going to work on something else instead.