Should you leave your PC on or off? Well, it depends on if the PC (personal computer) is in a home or work environment.
Do you know how the time is kept correctly on your computer even when it is off for a few days? It is all due to CMOS Batteries that are placed on the motherboard. Thees batteries, typically non-rechargeable CR2032 versus rechargeable LR2032, last a period of three years which is often the warranty length of the unit. The more times the PC is kept off, the more times the PC has to depend on the CMOS batteries thus reducing the lifespan of the batteries and unfortunately, when the battery fails, it will cause error messages to display during POST. Sure you can press a button on the keyboard to continue but if you need to restart a PC remotely, someone still has to go to the machine and hit a key to continue unless you setup a robot arm as well.
Leaving a computer on also give the OS (such as Microsoft Windows) the opportunity to search, download and install updates. You antivirus program will also be able to install new definitions and scan your data and programs as well. Keep in mind that HP LaserJet 1022 and Windows Update on Windows 7 at times causes issues so set Windows updates on manual instead of automatic.