Think of it like what you do to change the oil on a car.
Downloading the update is like driving to the store, getting your new oil and filter, and driving to your garage, now you have your car and oil.
But now to swap out the old oil with the new oil, you need to turn off the engine.
Computer updates are similar, downloading the update is like getting your oil and all your tools and supplies ready.
But a lot of the underlying code/files cannot just be swapped on the fly, because the OS is constantly reading from these directories, and will simply crash if you swap them. Kind of like if you just pull the oil filter off your car without shutting off the engine.
So you need to turn the computer off so it can replace all these files while the OS is not using them.
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