Why use 24 volt and not 12 volt?

Answer 1

More cranking power.

The same reason, if your trying to fill up a swiming pool. The more or higher pressure of flow of water will fill the pool up faster... more water in less amount of time. More voltage more flow of electrons in a given time and the higher the voltage the lower the amperage draw or current pull... you could use a smaller wire and not worry about it overheating and failing. theres a lot to take in consideration for the application.

The military uses 24v systems for several reasons. One is for cranking--and when big diesels started using electric starters, those starters were all 24v. (New heavy-duty diesels use 12v starters.) Another is for the radios we used to have. The old AN/VRC-12 series radios (VRC-12, VRC-46, VRC-47 among them) had vacuum tubes in the power amplifiers--the radios were designed in the 1960s, when they didn't have transistorized final amplifiers as big as the Army wanted. Tubes need high voltage to work, and it's easier to get to 350v if you start at 24v than if you start at 12v. The current generation of radio has a transistorized amp in it. The real reason they stick with 24v is commonality: if they went to 12v systems the warehouses full of truck parts, communications gear and everything else that can be used in a vehicle would work on only part of the fleet. They don't want a situation where they issue a troop a 12v radio to install in a 24v vehicle and the radio fries, or where they issue him a 24v chemical agent alarm to install in a 12v vehicle and his whole outfit dies from the nerve gas attack the alarm didn't tell them about.

For large vehicles using a lot of power especially for cranking the engine, 24 v is used because on 12 v the connecting wires would have to be of a heavier gauge . . . copper is expensive.