This is my response to the question - "should I leave my computer on all the time?" The short obvious answer should be no, just like you don't leave your car running in the garage, you don't need to leave a computer running all the time. The reasons require a more detailed response and a bit of a history lesson.
First - the why not. Every computing device is rated in MTBF or Mean Time Before Failure. This is a measurement rated in hours of time. For example, your hard drive might be rated at 10,000MTBF or 10,000 hours before it wears out. This is not a guarantee, but the mean (similar to average) time that the device is likely to last. Since your computer is a collection of devices, each with their own "timer" running downward, the longer you keep the computer running, the less life you have left.
The exception would be if you have something automated that has to run during a time that you are not using the computer. A business might have batch processing happening at night, a day trader may be pulling stock quotes from Tokyo to calculate buy sell prices for Wall street in the morning, or other such activity requiring off hour usage. If the computer is just sitting there, the sands of time are running out and you are potentially exposing it to hackers.
If you are on high-speed access to the Internet, the longer the computer is running, the more accessible it becomes. While you sleep for 8 hours, a porn site could be run from your computer, it could be used to hack into a government agency, or maybe only steal your identity. Eight hours is a long time to allow a hackers computer to find your armor's chink and exploit any weaknesses.
"But someone that worked in the electronics business for 20 years told me a computer should be left on all the time" My answer to that is simple, they told you that many years ago or they haven't been in the business for 10 years. In the past, the components in a computer were much larger and the solder joints (shiny metal around the "do-dads" on the computer parts) were huge. When on, the solder joints heated up. When turned off, they cooled down. This had the effect of bending a piece of metal back and forth till it breaks. Therefore in the old days, it was better to leave computers running all the time.
These days the solder is so small and the techniques and material used has improved so much that the temperature differential between on and off no longer has a dramatic impact that it did ten years ago. Any temperature difference still impacts, but you will likely replace a computer before the solder joints fail today.