For me, KRunner has some quick actions for certain applications that I use a lot. Like a quick connect to a VPN service.

Krunner is better at displaying longer search results. The launcher truncates anything longer than 1 line. Dictionary definitions are an example.

Krunner also keeps what was last typed in, which is very useful if I'm adding up a bunch of numbers, or if I need to switch to a different application before using the result.

Think it’s mainly just preference. I prefer krunner since it feels closer to raycast when I was on Mac, then when I switched to Linux as my daily driver, runner just felt natural. (Tried out Alfred for a bit but krunner did everything I personally needed)

KDE and RAM usage...

i.e. it does not use enough RAM

When I first switched to kde 6. Krunner had to replace andromeda launcher for me cause it wasn't updated. now though? Krunner is just ingrained to my muscle memory and workflow I can't turn back to a launcher

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.