Because I do answer some anon questions, and I understand that some people are shy when asking a personal question, giving a confession, asking for advice, or making a compliment and they want to be anonymous. Also some of my friends in real life dont have tumblrs and they like to leave me a message sometimes.
But if it’s a simple question like “who is your fave celeb” or “what is your fave food” I don’t see any reason to need to be anoymous, and if I don’t feel like clogging up my page I will answer privately.