If you think about the question as part of learning German, it can make a lot of sense and be useful. A German says to you, "Haben Sie einen Stuhl?" and you think "ACK, oh my goodness what the heck is this guy asking me???" You can then say, thanks to Duolingo ;) "Was ist ein Stuhl?".
This exact question is used in philosophy a bit. See http://www.roangelo.net/logwitt/first-question-philosophy.html#general-definition-of-the-word-chair