https://www.bustle.com/articles/65251-bdsm-contracts-really-exist-im-a-sexual-submissive-and-heres-how-fifty-shades-got-them

Here is one if you happen find this interesting. Like i said before these "contracts" are there mainly to help both parties know what other person likes etc (hard and soft limits, safe words etc). I think most people don't really use them, they discuss these things verbally, but writing it down also help you to really think about it.