Why we use 4-20 mA in analog signals?

The 4-20 ma signal dropped across a 250 or 500 ohm resistor creates very convenient 1-5V and 2-10V voltage ranges, respectively, also fairly standard in the industry and works off 12V battery well. I read in the nuclear sunami in Japan, the techs pulled car batteries to keep sensors hot!

Actually, you can use any range you choose! But if you do, you will have to design and manufacture both the transmitter and receiving controller to match.
The 4-20 ma range is a "standard" adopted by tradition and also ANSI 50.1, and most process instruments and controllers use it. It was developed during a time when teletypewriters used a 20 ma current loop for communications, so parts for that type of circuit were readily available. There once was a 10-50 ma current loop, but it has pretty much faded out.

4-20ma advantages:
The 4 ma "bottom of span" signal allows the receiver to detect a broken wire. Since the loop current never falls to zero, can be loop powered, allowing for two-wire. 20 ma is low enough to be made intrinsically safe, in most locations.
The constant-current feature of a current loop cancels out voltage drop errors due to long wiring runs (of course this would also be true if you selected different current values for zero and span).