I can't for the life of me see any good reason to leave a PC powered up when it is not doing anything.

I usually shut down because every HD has a life span.  However on occasion (every few weeks) I'll close everything down but leave it on overnight for updates etc.

I will also qualify my response above:
My PC is in hibernation mode all night and sleep sometimes during the days.
I don't have problems with UPS, they only deliver packages during the day

If I was on a more/less fixed schedule, I would prolly shut everything down at quitting time, with a fresh startup as part of my morning routine.
 
The reality for me is that I have no set schedule.  I grab DAW time when I can - frequently in the wee hours.  There is a bit of hold-over from the old days, and prolly N/A now,  where power cycling electronic equipment reduced its life expectancy.

My rather long-in-the-tooth desktop DAW (I think it's about 10 years old and still chugging along and amazingly still doing what I need it to do, which is a good thing as I'm looking at a bit of a lean year and not up for a full replacement) gets shut down every night for sure. That's partly because I sleep in a loft bed above it and don't care for the noise and electrical energy pulsing away beneath me while I sleep. My long-suffering wife wouldn't stand for it either - even she has limits. Also it has quirks and won't stand for sleep or hibernation - I've learned not to try, it just goes into a panic reboot loop that is nerve-wracking. But like someone said above, I've been doing it this way for a long time and the system seems to be relatively happy and stable despite its advanced age, so I have limited motivation to change what's been working so far... 

HARDDRlVER
I was wondering if you completely turn off your computer(s) after you're finished with a session.
To be specific, whoever answers, can you state what you're running, a desktop or laptop, please.
Any issues?

I have a laptop. I've always left it on. When I installed splat, I continued doing so. One morning I went in to continue with a session and the laptop was frozen.
Ctrl/alt/del did nothing. Pulling USB connections...nothing. only option was to hard shut down. When I rebooted (alt few times on purpose to make sure everything sat well), the performance meters were acting up. I was/altm getting a great deal of latency clicks/pops. I've gone through everything in the Sweetwater latency advisory. Nothing will fix it. If I start a brand new project...one audio, one midi, and nothing is even recorded yet, those performance meters are going wild. The latency program I downloaded a few weeks ago, doesn't look anything like I'm used to seeing.
I think my only option is to once again, reinstall windows. Those people at cakewalk and waves are gonna think I'm up to something, reinstalling the software so many times

Re-installing windows might not help here. You could have a physical malfunction, such as HD failure, bad ram or it's overheating because the fans have died. 

Laptop here
 
I always shut it right down when I'm done... I've never trusted sleep mode... Don't ask me why.

synkrotron
Laptop here
 
I always shut it right down when I'm done... I've never trusted sleep mode... Don't ask me why.

The C6/C7 power saving sleep modes for Haswell chips will cause some power supplies to trigger circuit protection and shut down when coming out of sleep mode. Then you have to do a hard restart of the power supply. I read about this when building my last computer, and since my power supply wasn't on the list of tested and approved models, I just disabled C6/C7 in BIOS. But I'm like you, sleep/hibernation/power saving modes have tended to give me problems over the years, so I don't use them.
 
 Haswell Sleep States
 

I have both a laptop and desktop and now turn them both off at the end of the session.  I used to leave them running and noticed a couple of BAD things.
 
1) After a few years of leaving the desktop running the HDD went bad. The second time this happened I thought that leaving the unit on 24/7 might have something to do with this.  (this was on 2 different desktops).
 
2) I noticed that my electric bill went down about $20 a month once I started shutting down after sessions.  The bad thing was it was costing me over $300 a year leaving the units on.
 
I use my laptop as a GUITAR AMP and feed the output to SPLAT, I usually just put this to sleep by closing the lid and not doing a complete shutdown. I just need to unplug and replug the USB cable for the McMillan foot controller.  Win 10 loses the connection coming out of sleep mode.
 
Jim

Desktops, either Windows or Linux, stay on 24/365 other than being restarted once in while to install updates etc., a forced reset following a freeze or in the case of Windows, a soft reboot once a month or so just to clear out the RAM and restart services. They sleep when not in use so scheduled tasks like backups etc. get done overnight or so shared resources are available on demand (one is a media, email etc. server).

A UPS ensures a tidy shut-down in the event of mains electricity failure.

My MacBook Pro does the same when I'm at home. No need for a UPS, the built-in battery does the job instead. If I've taken it somewhere I usually shut it down to save battery. Battery life in an i7 Retina MacBook needs all the help it can get.

As for HDDs, or any other electronics, wearing out if left on, the thermal changes caused by the circuits powering on and off can cause long-term reliability problems as well.