We have a contract and for us it is a powerful symbol of where we are and what we want out of our D/s relationship. We looked at the ones online but they were too generic for us, so my Dom decided to write ours so that it says in detail exactly what we want it to say.
Yes, of course we could have the same relationship without the contract. It is not legal or binding in any sense other then our word to each other. However, it is powerful to us because it was written with great care for whom and what we are to each other.
We review our contract every 3 months and will for our first year. When that year is up we hope to have a new contract that consist of a couple of lines that state very clearly what we want and expect out of each other. We will also have a collaring ceremony and I will receive my permanent collar.
My Dom likes rituals and since being with him I have grown fond of them myself. For us symbolism and rituals are a useful tool in keeping our D/s alive and thriving.