Apple - Is shutting down an iMac or Mac mini really necessary?
The benefits of shutting down vs. sleeping is an oft-asked question. Over time, reasons to prefer shutting down have either been resolved or significantly ameliorated.
For instance:
It used to be that shutting down early macOS (then OS X) machines caused issues with timed log clean-up routines. This has been resolved with the OS recognizing when routines haven't run and rescheduling them.
Similarly, long-running systems used to exhibit instability relating to memory issues. Shutting down the computer cleared the memory, temporarily resetting things. However, memory management has subsequently improved in later versions of macOS and this is no longer a significant issue.
A shut down system used to be significantly cheaper. However, advances in hardware and software have improved the ability of Macs to sleep efficiently. In 2020, a 27" iMac with Retina 5K draws 0.27W when shutdown and 1.25W when asleep (compared to 40W when idle) (source). Apple prepares environmental reports on all its devices extending back several years that you can check. Copies are available from the Environment section of Apple's website.
In addition to these changes, features like Power Nap improve the utility of sleeping Macs.
For me, I set automated sleep and wake times and almost never reboot.
Can't believe almost every one of these recommendations is saying to NOT restart your computer on even a semi-regular basis. I have a rMBP which I restart at least once a week. I also have a Early 2008 iMac that acts as a server that I restart at least twice a month. I do this b/c it allows the machine to purge unused memory and temp data that builds up over time.
I work in IT for a large, Mac-based school district. I cannot tell you the number of times that a simple restart fixes simple issues and how much better my staff's computers run if they restart once a week. On average, especially if your machine gets used quite a bit; I would recommend restarting your Mac (or any computer for that matter) at least once a week. A lot of it depends on how you use your Mac.
In the end, if you're Mac isn't a server, then restart it every once in awhile. "Always-on" is meant to be for Servers, and even then, servers have to be restarted from time to time.
No, with the exception of restarts for software updates, shutting down your iMac or Mac Mini is not necessary. You are quite correct that you can put your device to sleep for long periods of time or even leave your device up and operating without causing any damage to the device, so long as proper precaution is taken to ensure surge protection for your electronics.
An even better solution would be to provide some form of backup power supply with a USB link to your device that works in OSX so that when a power loss is detected the device will shut down properly for you to be able to resume your uptime later. Most servers or datacenters have operated this way for years already, and the difference between the service life of servers and home hardware has come increasingly closer over the last decade. The days of the recommendation being to shut your PC down whenever you aren't at it are becoming increasingly out-of-date.