Hi!

Can someone PLEASE tell me why standby mode is used?

There are a few reasons.

1. It allows for easy addition of a remote control circuit. By leaving the
microcontroller and a sensor active, a remote is easily added to a design.
2. It removes the need for a separate memory device and a means to power
it...at least until the power goes out!
3. Some designs use it for a quicker start-up time.
4. A device can be turned on from standby using a special "power up" command
or timer event. WebTV boxes do this.

Why should such appliances be left in standby mode instead of turned
off?

For the ones with volatile memories backed by the power line and a portion
of the power supply, the answer is obvious...remove power entirely and the
memory goes away. For others with remote controls the answer is also
obvious...something has to be on and "watching" for remote power-on signals.

For some devices there is no good reason and unplugging them won't harm a
thing.

I don't know if turn-on cycles without the "aid" of a standby supply are
more stressful to the other electronics in a device or not. Given that most
everything is turned off and started "cold" by the few parts that do remain
on, I would have my doubts that standby modes could increase the life of a
device.

Devices with "hard" on-off switches are usually a little bit better
protected than those which are always "on" or in standby. This does depend
on the location of the switch though.

If the appliance is turned off instead of being put into standby mode,
will the appliance's life be shortened, by how much and why?

I doubt that it would be. There are a lot of factors here. First, some
devices will fail in standby because the wear on a small part of the device
is constant. This can lead to dried up capacitors or other failed
components.

Starting any device is a stressful time on all the components involved.
Things are starting up and stabilizing. Usually this doesn't affect the
device's life significantly as you've got to turn on it on sometime!

William