I've yet to hear the most OBVIOUS question

And yet it begs to be asked - WHY did Rob Goldstone write the e-mail letter to DJT, jr in the first place?  

It looks like blatant entrapment, clear cut bait & switch.  Seems to me that DJT jr & Jared were played by Goldstone - with a possible assist from Manafort, because why else would a super savvy political operative with full knowledge of where the lines are that one does NOT step over not squash the situation from the get go.