I helped the user @Natsu to solve his employment test. I spent half a day on it, because there were many things that I did not knew myself. So I posted my answer and he commented that it solved his problem!
But then an hour later he deleted his question and my whole work was pointless! I answered the question not only for him, but for many other users who will have the same question in the future.
Why can users delete their questions which already have an answer, wasting the helpers time? I won't help any fresh users anymore after this, thats for sure.