This question is often asked by New boards. After 1-2 years boards do not ask such questions because of the monotony of the answers received - unless they have an extraordinary profile with guys quitting 150K+ USD jobs to join Civil Services.
So how will you answer this question?
Why do you want to join Civil Services/ IAS?
Note that there is a standard reply to it, and then there is a bad reply to it - that cost you marks. Also, reply to this thread only to give an answer that you would give to the board.#interview-discussion