Something I've wondered for a while but never really found a satisfying answer to - what exactly is it that Windows does in the background whilst the PC is idle?

I've noticed e.g. by having task manager open, or by checking a wall power meter if one happens to be attached, that after a few minutes, something starts churning in the background, usually stopping withing seconds of touching the mouse/keyboard (which in itself makes it tricky to find the culprit).

I can think of a few things that could be scheduled in the background e.g. defrag/TRIM, AV scan, .NET optimisation after an update, pre/superfetch - but I can't think of why they'd need to run so frequently? It just seems wasteful to keep drawing power doing something which is of no apparent benefit, especially for laptops.

If anyone knows what it is, is it possible, like for .NET, to force-run whatever task it is so it stops churning away?