A Raspberry Pi 3 Model B
It's connected to my 3d printer and runs octoprint allowing me to upload print jobs. and control the printer from my home network.
It serves up the Pi camera video stream.
It can also switch the printers light on and off.
No cluster setup.
Many programs do stuff once during startup that they never do again, sometimes creating redundant data objects that will never get accessed in the configuration its being run in. Eventually the kernel memory manager figures out that some pages are never used but it can't just delete them. If swap is enabled it can swap them to disk instead. It frees up that RAM for something more important. It's usually minor but every few MB helps.