Why do interviewers ask 'what other jobs have you applied for?'?

and how should you answer it?
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just to see whether or not you are really serious about applying the job
If you are really serious then you would probably apply for a similar position elsewhere. some people will answer nowhere, but i think its too unrealistic to say that.

I will tell him/her the truth and explain that I need to live (in a polite way)

and that way they can see that i am mature and ready for this position

plus doing so will give you a competitive edge. meaning if they decide not to hire you, others will.

But then if you are really passionate about entering the company then you should tell them that other jobs are just backup plan. But then you must always explain why you love their company so much