So why are protocols used in D/s relationships?

There are as many answers to this question as there are people using them. However, the most common reasons appear to be that the protocol:

• creates a clear distinction between the Dominant and the slave
• provides a good foundation for the relationship
• through its rules clarifies for the slave how to serve and behave in different situations
• strengthens the bond between Domnant and slave by distinguishing them and their unique relationship from others

Most protocols are directed towards the slave. They often regulate the slave's behavior, communication, and interaction in one way or the other. 

Protocols can be very simple or very detailed. They can prescribe the specific way in which certain tasks have to be performed. The slave might have to use specific honorifics and figures of speech for addressing his Dominant. Each individual task or action might have its own rules. Clearly, the more specialised the rules are, the more difficult they become to learn as well as to enforce. When the number of rules increases (and often it does if one tries to provide rules for every possible situation) the protocol becomes cumbersome and almost impossible to keep track of. The slave will have difficulty learning and separating the numerous rules for all the different situations. And yes, it also becomes increasingly more difficult for the Dominant to reinforce and correct all mistakes.