I leave my Mac Mini and MBP on 24/7 unless I update them... then they get restarted. First of all they go into sleep mode and mostly power themselves down to using minimal energy anyway so there is very little if any wear and tear or energy usage in it's sleep state. Second there is the issue of powering you computer off and on all the time which heats up and cools off the internal components, power supply, caps, etc... which causes a range of thermal expansion from an extreme cold state when it's off to a heated state when fully booted.... which in itself can cause excessive wear and tear.
The answer is that if you use your desktop computer at times throughout the day and most days of the week you can leave it on. Make sure it's plugged into a high quality computer grade surge protector or high quality battery backup... which in turn is plugged directly into a good outlet. If you only use your computer occasionally... several days a week and not every day then you can turn it off in between days. If you are in an area or if it is a time of the year prone to brown outs, outages, power surges, excessive heat waves, bad weather... lighting strikes, etc... you may to want turn it off in between uses and/or use a very high quality battery backup with surge protection.
I have PCs plugged into APC server grade battery backups that I left on 24/7 that have lasted almost 20 years (I did change the main power supplies and the hard drives a few times but the rest of the system board and components were the same). My current 2012 i7 Mac Minis used for business are on 24/7 with an APC backup as well.