A short answer is generally regarded as more emphatic and polite than just an abrupt "yes" or "no".
When students are learning English, the teacher will encourage the longest answer possible. This is so that the students will have practice in construction and pronunciation of the language.
Are you on WhatsApp?
Yes, I am.
No, I am not.
Yes, I am on WhatsApp.
No, I am not on WhatsApp.
Do you use WhatApp?
Yes, I do.
No, I don't.
Yes, I use WhatsApp.
No, I don't use WhatsApp.
You should read:
(1) SWAN, Michael. Practical English Usage
. Oxford University Press, 1995.
(2) BOURKE, Kenna. Test it, Fix it - Vocabulary Pre-intermediate
. Oxford University Press. 2006.