well there are many reasons why i suggest shutting down the computer when not in use..
there are numerous myths as to why you should leave it on.. and some people get confused between shutting down to totally cutting the power to it.. and shutdown standby vs simply standby.
First off.... no one in their right mind would leave their car idling in the garage or outside 24/7.... while it's totally innefficent to do so.. that's not argued... what is argued sometimes is, what's easiest on the car.
The fact is that everytime you go to start your car, there is obviously some startup related "wear" that occurs each time... it's hard on vehicles to repeatedly shut off and start frequently through the day....a good example is many shippers doing diliveries, is actually statistically going to do more damage and cost the company more (or themselves) by shutting the vehicle off at every drop point (unless they are going to be there for an hour or more usually obviously).... sure they are burning fuel.. but in the end the cooling and reheating that takes place between off and on WILL/DOES always do things, not to mention all the other related parts of the vehicle. (i'm also a light duty and heavy duty mechanic "somewhat"...)... Many of these things are directly related and can be discussed about computers...
Now leaving your car running for extended periods of times can and does do damage when not in use... idling is typically somewhat the hardest thing on a vehicle.... not true for a computer... BUT what is hard on both is things in both have basically an HOURs of life expected... For example even though there are few moving physical parts in the computer, fans and standard hardisks DO have a lifespan.... motors wear out and die/burn up.
Top it off, if something happens to go wrong while your away for long periods of time..... this can result in the total destruction of a computer in the event that it fails to shutdown and stay shutdown, most machines will continuously reboot which CAN and i've personally seen, resulting in TOTAL disk corruption or physical damage to the drive or other parts of the machine that happened to fail.
Anothing thing is.... some people that shutdown there computer have a habit of shutting down followed by flipping the switch in the back AND/OR flipping a switch on their surge bar/pulling the plug.
THIS is extremely hard on the eletronic components in the system and should be kept to a minimum. The only time you should ever flip the PSU switch or the surge bar switch/unplug is when your doing major changes to the system, such as cmos resets or replaceing a video card.. (generally it's still wise to leave things plugged in so that at least the ground still continues to work as a valid grounding point for static discharge)
Another thing is people buying surge bars... specially cheap ones... This is understandable mistake yet completely useless.
I can ask majority of the public one simple question.
When was the last time you EVER had to reset a surge bar?
So far..... i've never gotten a responce saying YES excluding one which was due to overloading the circut unrelated to a computer....
Surge bars are mostly a joke.. they are designed to typically trip at an extremely high voltage point... something that modern PSU are designed and can easily cope with dumping excessive voltages/power quite often through the ground or other methods of curbing this issue. (at least with good quality PSUs)..
What KILLS a computer fast is undersurges/brown outs/LOW voltages. These really really really hurt computers. droop/spikes factor in obviously but there is only so much eletronic components can take before they fail.
The only way to protect against these is to invest in a UPS.... and trust me.... for at least a DECENT UPS... personally i prefer APC.. something in the 600VA range as a minimum is plenty for most people and will provide a good long lasting use. I've a ups that is almost 10 years old that still holds a great charge.. (it's always a good idea to do a battery drain on them every once in awhile)...
Really i could go on and on about different subjects surrounding the computer systems.. but it's alread 2am here and i intended to head to bed at 12am lol..