Applied for PhD, professor asked me to disclose offers from other universities, why?
I don't have a definitive answer, but I can offer some interpretion: with such a question a potential employer, not matter whether academic or not, is probing how likely you are to accept an offer if you were to receive one. 

What I suggest in this circumstances is to tell you have other offers, if you did receive them - it will make you look attractive - but not specify them. After that, hint to the fact that your other offers are not your first choice, unlike the present one - that will make you look committed. 

Of course all of the above stands if you did receive other offers. Honesty first. But no need to be honest about what is your favorite choice, as it's probably impossible to be honest about it, at least until you know what the offers are anyway!