A reference implementation is a function or feature in either hardware or software recommended by someone, that, if implemented, will help in some way.
The current specification documents are complex and wide-ranging. So, even making a small implementation is a hard judgement to make just by going off of what is written in a specification document. With specification documents being worked on up until the moment of implementation, it becomes difficult to prioritize them as well.
When it is time to make an implementation to the current system, if a successful reference implementation was performed, those choosing to make the implementation can be more confident in their decision.