The protocol is the detailed plan for your systematic review. It explains your rationale and methodology, and should be developed right at the beginning. You can refine it as you go on.
Publishing your protocol in either a register or a journal will help avoid duplication - you don’t want other researchers doing the same review as you. It also “…reduces the impact of review authors’ biases, promotes transparency of methods and processes ... and allows peer review of the planned methods.” Cochrane Handbook, Part 2, Chapter 1.5
If you need more convincing, here are some further arguments for a registered protocol:
It’s also a good idea to check through protocol registers to see if another group of researchers has already registered a review on your topic. (See the "where" box below).