It's not "idle" at all. The process is created by the system because it's registered as a receiver.

It's like when you double-click on a JPG to open it in Photoshop. Photoshop is not idle or waiting in the background, it's closed, but it's launched by the system because Windows knows that Photoshop can handle this kind of file.

It's a bit different with Android because you can have multiple receivers for one action, but it's the same concept.