Sounds like you didn't 'get it'.
I worked industrial electronics constant current for most of my life.
We started in the 1970's with 10 - 50ma, but everybody eventually went with the 4-20ma type.
You cannot use (as *steve* & BobK tried to help you figure-out) a zero ma signal as
your zero value point for the current signal, because if the circuit failed, you would also
read zero mA, and you would not know that the circuit failed, you would just think that
your circuit was alright, and you were actually reading a 0ma signal.
The circuits you're wondering about, use the 4ma value as the signal 'zero', because
you know the circuit is working when you read 4ma. If the circuit ever does actually
read 0mA, you KNOW the circuit is not working.
Does that explain it?