The higher the voltage, the more energy can be transferred for the amount of current (voltage is like pressure, current is like flow rate). People use higher voltages so the current drops and they can use thinner, cheaper cable, and because the lower current flow causes less energy to be lost as heat (hence electricity pylons running at very high voltages - they need to transfer a lot of energy without overheating or being wasteful).
For the vehicles, theres no need to change unless theres something wrong and you cant fix it. Its difficult to find 6volt kit for replacements, and with lower current, 12 volts and 24 volts are more resilient to bad connections / faulty components restricting the flow.
Give us some idea of the problem and we'll try and help you fix it instead of converting if you like.