You asked for some practical applications of current loops. Here are a few. Some are historic, and some are still being used today.
Early Teletype machines, like the Model 15, used 60 mA current loops between machines. Later models, like the Model 33, used 20 mA loops. The advantage in both cases is that you could run lines for several miles between machines without the need for any repeaters, since the constant current overcame any losses due to the resistance of the lines. Of course the voltage drop across these distances increased as the distance increased, and some lines were operated at supply voltages up to 125V.
Another advantage is that you could add additional machines in series with the others anywhere in the loop, and the power supply will automatically compensate by raising the voltage driving the loop.
These Teletype loops used an absence of current for a "space" condition, and the presence of current in the line for a "mark". Since a spacing condition (no data) was the default condition, this reduced power consumption in the power supply circuits most of the time.
Model 33 Teletype machines were widely used as computer terminals for minicomputers in the 1970s-1980s, and thus most of them came with a 20 mA interface. Even the original serial card for the IBM PC had provisions for a current loop interface.
MIDI is another example of a current loop interface. It uses 5 mA.
Another type of current loop was and still is being used in some places for instrumentation. It is called 4-20 mA current loop (10-50 mA has also been used). Unlike the constant current in the loops discussed above for sending digital data, the 4-20 mA loops are used to convey instrument readings such as pressure, temperature, level, flow, pH or other process variables. Usually 4 mA represents a reading of 0, and 20 mA represent a full scale reading. So if the full scale of an instrument was 160, each 100 µA increase in current would represent an increase of one in the reading.
A device known as a Transmitter is used to convert the reading into a varying current. Modern ones are rather complex.
Like the 20 mA and 60 mA digital loops, an advantage of the 4-20 mA current loops is that they could be run over a telephone pair, for example, for long distances.
The reason they started with 4 mA instead of 0 mA, is the latter was used to indicate a fault (open loop).