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Like any standard the purpose is to allow others to interface with a control, sensor, etc. If I am not mistaken 4 ma thru a specific resistor is 1 volt and 20 ma is 5 volts - the resistor is 250 ohms. One standard is 1 to 5 volts but it seems that either 1 to 10 or 2 to 10 volts is more common.
Like other standards, they probably were convenient round numbers or were the standards adopted by a popular manufacturer some time ago and everyone else just followed.