Precious seconds wasted

The reason for a clean shutdown is to permit flushing data to the disk. By the time that the OS has gone down, that's happened. One can just shut off the power once that shutdown has completed.
The largest issue with unclean shutdowns was that at one point in time, the filesystems used (FAT on Windows, HFS on MacOS, ext2 on Linux) could become corrupt if power was cut at arbitrary times. This has been addressed with later filesystems. Only FAT still sees much use, principally on USB flash drives and SD cards, due to the ability of many operating systems and devices to understand it. While some userspace software can deal poorly with having power cut -- I particularly wouldn't do it while updating an OS -- generally shutting down a computer uncleanly isn't nearly as problematic any more.
What's the cost of leaving a computer on, some small amount of power usage? A reboot will log a user out, so there's no risk of leaving an unattended, logged-in computer. I have my monitor power off, but I don't normally shut down my desktop when leaving it unattended.
- It depends on the computer, but the power usage could easily be 250W+. While not a ton of power, it adds up quickly.
  But that's only if you don't have your computer set to sleep/hibernate
  
  Idle power is not usually that high unless you are talking about a multi socket server.
  A gaming PC is usually less than 100W and an office PC is usually less than 25W at idle.
  
  but the power usage could easily be 250W+
  I mean, a beefy GPU could be ~400W, and a beefy CPU another ~200W. But that's peak draw from those components, which are designed to drastically reduce power consumption if they aren't actually under load. You don't have to power down the components in sleep/hibernation to achieve that -- they can already reduce runtime power themselves. One shouldn't normally have software significantly loading those (especially after a reboot). If you've got something that is doing crunching in idle time to that degree, like, I don't know, SETI@Home or something, then you probably don't want it shut off.
  The reason fans can "spin up" on the CPU and the GPU when they're under load is because they're dissipating much more heat, which is because they're drawing much more power.

UPS triggers a shutdown, OS starts installing updates.
The shutdown command was a warning, not a request.

What the hell is he watching in the first panel?
- Y'all Tube. Says it in the second panel.
  I think he's just got red cheeks from the alcoholism that also contributes to his anger problems
  
  Case closed, thanks Holmes

Just wait till it gets to bios and hit the power button

Or you close and the fookin Windows update commences.
- OK so my options are Sleep, Restart, Shut down, Update & Restart and Update & Shut down. Let's think about this carefully, I'm in a hurry so I need to shut down right away. Let's select Shut down.
  Windows 11: Restarting, Installing Windows Updates, Restarting again, Installing even more updates. One final restart and just finishing things up (whatever that's supposed to be). And bam here is your login screen, you are welcome.
  Then Bill Gates busts down the door and goes LMAO GOTTEM

Why does every single os out there place these buttons next to each other?
- Because it's common to group things
- In KDE it depends on your Plasma theme. Often they are next to each other but it's no problem if you misclick anyway – there's a confirmation screen with an auto-accept timer so you can just undo the wrong action and choose the right one.
  
  KDE FTW!!!
- every single os
  I use sway on Linux. I don't have shutdown or reboot menus. When I shut down the computer via software, it looks like this:
  
  # shutdown -h now
  Rebooting is pretty different, looks like this:
  
  # reboot
  I guess that my case has a physical reset and power button that are -- while different sizes on mine -- near each other. You could put a button wherever you want to, though -- those things just short two pins. Go to Mouser Electronics or Digikey or something and get one of those big fancy E-Stop buttons with a protective shield that you have to flip up and run the leads to the reset or power pins, put them on opposite sides of the case, and you can use that. I think I remember reading about someone who used a key switch (like, you have to physically insert a metal key to twist the thing and flip the switch) to power on his computer, just for the aesthetic.
  EDIT: Apparently that shield is called a molly-guard and the term was actually originally from computer power switches, prior to making its way to other industrial hardware:
  molly-guard
  Originally a Plexiglas cover improvised for the Big Red Switch on an IBM 4341 mainframe after a programmer's toddler daughter (named Molly) tripped it twice in one day. Later generalised to covers over stop/reset switches on disk drives and networking equipment.
  
  Rebooting is pretty different
  Not if you use shutdown -r.
- Careful what you wish for or they’re gonna add “slide to shut down” to desktop PCs.

Just leave it on. Lock the screen and walk away.

Just plug out the power cord, fuck it!
- This guy doesn't park his hard drive.

My pc restarts in like 15 seconds at most.
- Mine usually does too, but every once in a while, my Ryzen system will randomly decide that now's a good time to retrain the RAM, and I gotta wait 3½ minutes. Which feels like a fucking eternity when you're just trying to shut down the PC.

Laughing wit my NVMe PCIe 4.0 SSDs in Raid 0
- Raid 0 is fun. It's like if two gunfighters had a suicide pact.