I think the idea of a contract or vows that are to be kept in a D/s relationship can be very comforting and as well as stimulating.

There is a level of security in knowing that your partner and you have agreed on a set of boundaries within the relationship. If those boundaries are broken, then the couple can assess the relationship and determine whether or not to continue.

Contracts that leave all of the decisions with one party get my no vote, because I'm not into non-consensual activites and I see them as a step closer to abuse rather than a step closer to a relationship based on mutual trust.

A marriage vow and legal status are a wonderful way for a couple to cement their d/s relationship.

However, for same sex or poly couples, that same level of legal protection isn't available worldwide. Also, not every sub and dom wish to get married.

Contracts like anything else, are another tool for the relationship. Use them or don't, it's your choice.