4-20mA Current Loops
4-20mA loops are used a lot in industrial environments.  The current signal is generally less susceptible to noise and signal loss over long cables.  It's especially popular for things like transducers, where the transducer is actually powered with the signal line.  This allows a 2 wire connection, where one line supplies the power rail (typically 24V) and the current returned is based on the measured value (temp, pressure, etc.).  This can help save money on wire in a large plant, but most hobbyist probably don't care about such things or need to worry about electrically noisy environments.  4mA allows a minimum current draw by the transducer even if there is no signal.  It also provides trouble shooting capability, allowing to distinguish between a signal of 0 and a lost connection or powered down unit.  I don't know where 20mA came from, but 20mA is easily dropped over a 500 Ohm resistor for a 0-10V signal or over a 250 Ohm resistor for a 0-5V signal.