It really depends on the situation. Sometimes asking "what have you tried?" is indeed useless.
However, a lot of time it does give us some useful information.
- By showing what the questioner has tried. One get a grip on the questioner's level of mathematical sophistication. One can taylor make an answer which the questioner can understand.
- Another very common case is the questioner misunderstand some basic concept. If the questioner doesn't show what has he/she tried, we will never know what the questioner is missing. No matter how good an answer is, it won't help the questioner until his/her basic mis-undertanding is corrected.
Of course, the real reason that most people asking "what have you tried?" simply because there are too many "problem statement question".
Remember, people here doesn't get paid by answering question.
- Why should we spoon feed answers to those who just want an answer instead of learning mathematics?
- Why should we help those who don't want to help themselves?