A protocol should state your rationale, hypothesis, and methodology. It is supposed to outline whatever the planned framework is for the evidence synthesis. You use the protocol as a guide to conduct the research.
It is recommended that a team registers their protocol after it has been decided upon and created. This is so others can see the team is working on it. Viewers could potentially provide contact information for possible collaborators.
Registering a protocol will also make it so a DOI (resource identifier) is assigned. This makes the protocol easy to cite and link back to.
It also establishes a location wherein you can publish supporting materials.
Lastly, it's important to note that some publishers require the prior registration of a protocol.