What should the purpose of a draft of a research paper be?
The draft itself is only the term for something that is surely not finished. The purpose depends on what you intend to do with it.


Drafts are often shared with collaborators to have a common ground for further discussion, fixing some notations and in general keeping them up-to-date with the progress. 
Drafts can be just for personal use to keep track of ideas and what has already been done.
Draft is also the version of the paper that is 5 pages over the limit and you know you will need to make changes to fit in.


Draft writing is normally not the goal itself, instead it makes sense to think about what the draft will be used for and then write a document that is useful for this.