This sandbox is intended for saving drafts of long, complex answers, especially answers whose composition takes a long time. It serves to localize to one thread the front-page "bumps" caused by edits to drafts of such answers, so that they may be easily ignored. Also, it helps to guard against losing longly-composed answers due to system crashes.
To use this sandbox, look for a free answer below which says "This answer is free for anyone to use". If one exists then please use it. Else create a new answer. Start writing your answer there. Be sure to save a first draft quickly, to minimize the chance that that someone else concurrently tries to use the same free answer. Then, edit it as often as you like. When you reach a point that you'd like to post it to the intended target answer, simply copy it there and post it.
When your answer is complete and you no longer need the sandbox draft, please "clear" the draft by deleting all of the text and replacing it with the text "This answer is free for anyone to use". This is important because it eliminates TeX code (which may slow down MathJax rendering).