Imagine the following scenario. It is one of these nights with the inspiring silence and you are eager to do some serious work. You happily write some maths in your tex file and a smile is appearing at your face as you look at the excellent typesetting of LaTeX. You can easily project your thoughts to the 2-dimensional space of the paper, until you have to look up this special symbol which your favorite editor does not list. You are stuck. The best thing you can do is to draw it and show it to someone. Let us hope that this someone will be able to recognize this symbol. Oh, my bad… It is too late and you need a reply in order to go on! What are you going to do?
Hopefully, there is Detexify which recognizes your splendid drawing (thankfully, the font designers spent some more time on these things) and suggests the tex code of the symbol as well the package which contains it.