Earlier quoted context omitted.
What do they use the second phone number for?
(1) It's often cheaper/free to call numbers on the same network. (2) Business phone/family phone. Edit: speaking from my experience in Eastern Europe. I assume it's similar in India.
If you run a small business, you'll also want to connect on all networks, so your clients will have one less reason not to talk to you. (But watsapp has made this point moot recently.)