4-20 mA signals are possible to divide equally, 0-4 mA as 
close loop checking.

We use 4-20 mA range as a standard bcoz we know very well 
when there is no input i.e. output must be 4 mA otherwise 
instrument is giving wrong output(i.e. instrument not 
having proper calibration).
-Amit

There are basically 2 reasons of using 4-20ma
1. we can know whether the instrument is working properly 
or not. we can distinguish "live zero" and "dead band". if 
there is problem in instrument then also it is going to 
show zero and if there is no supply then  also it will show 
zero so we can differentiate this two.
2. 4-20ma is linear with 3-15psi .

Nilesh Upadhyay...

We use 4-20 mA range as a standard bcoz we know very well 
when there is no input i.e. output must be 4 mA otherwise 
instrument is giving wrong output

the 4mA zero is termed a "live zero". If 0mA was the zero 
point there would be no way of telling if the transducer 
was reading zero or in a fault condition eg. an open loop. 
with a 4mA zero an open loop would drive the indicator eg. 
a recorder into a minus figure (-25%). The 4 - 20mA scale 
is to give a calibration points for 0-25-50-75-100%. 
Because the start point is 4 the steps are 4mA.

First the question is why can't we use 3~15mA or other
range, here also we can check live zero and also we can go
step current like 3,6,9,12,15(i.e 0%,25%,50%,75%,100%).ok
The reason is  the transmitter voltage is 24Vdc , here we
can load max of 600 ohms, for this voltage & load better
range is 4~20.
Why 4 ~ 20 is better since we can convert easily 1~5Vdc by
put 250 ohms.All the controllers and DCS , PLCs are operated
by the input of 1~5Vdc (external or Internal convertion)