4-20mADC process variable transmitters have the advantage over other types of signal transmitters that they are "2-wire" instruments. They get their power from the same two wires used for the 4-20mADC signal. In otherwords, they do not require a separate power input. This makes them convenient.
As mentioned by others, the current analog signal is less susceptible to interference. However, the only signal available is the process variable. The newer digital type transmitters can send a lot more information, such as, the process varible say pressure, the temperature, the tag number of the transmitter, the transmitter's health, etc.
