its just one of the many questions the immigration agents use to assess whether or not to proceed with further questioning. I have been asked things like

what do I do
how much money do I have with me
do I carry food
how many bags do I have
why do you travel so much
why is your English so good (hispanic agent, I am hispanic too, I thought her accent was terrible.........)
what are all those papers???? (I put a bunch of confirmations, car rental reservations etc over the counter)
who do you know in xyz place
Houston HUH?? why dont you go to............Miami??? (answer , been there many times as you can see , doesnt even feel like the US, I dont like the place..........Johnson Space Center is not in Miami :P )

my favorite is

"are you seriously telling me you are traveling ALONE, to South Dakota???????????????? and you dont know anyone there???????????"

they can ask anything they feel like and depending on your response (and body language) figure out if you are being forthcoming or not