PC 104 or Small Embedded Boards

Some peripherals may require a +12V rail. Many (most? practically all??) do not. LCDs, for instance, may need 12V for the inverter. Or not. If the board brings out +12V to an external jack for connection to some standard inverter type, you'd better be feeding +12V in someplace (I don't want wasted space, noise and cost with an extra SMPS on the motherboard).

In some cases, the board itself may work better with +12V available. For instance, some of the boards we use are jumperable to use +12V, if available, for the audio amplifier.

Essentially because of the "PC" part of their name. PC/104 is, roughly said, a simplified plain old ISA PC in a different "form factor". To the best of my knowledge, PCs have had 12V supply lines ever since they hit the street over 20 years ago. And -12V and -5V, too...

Note that even today, all the power cables going to hard disks and other drives supply both +5 and +12 Volts.

Usage of all those lines by PC mainboards has varied over time. I guess they were originally meant to drive the RS232 ports.

Nowadays, the +12 V seem to be used primarily as input for voltage regulators that feed the CPU and/or AGP bus drivers. They use the 12V lines because taking those 80 Watts from the 3.3V line would require uncomfortably high currents --- cable resistance becomes a non-negligible factor at 25+ Amperes.

That rather obviously depends on what peripherals you're going to use. If they don't need 12V, your main board doesn't either.

Some may. You buy them --- so _you_ get to choose.

Most hard drives, at least, only use 5V though.

On PC/104 boards, at least the ones I've used, 12V is mostly just passthrough to peripherals.