Comment by basseq

  But I think Johnson is effectively acting as a compromise 
  candidate: a lot of Clinton voters really hate Trump and a 
  lot of Trump voters really hate Clinton, enough that they'd 
  both prefer Johnson over the other candidate, even if they 
  don't actually know much about him, or even mildly dislike 
  him.

That, and a lot of people were voting less for the man and more for an independent ticket. And not even with the expectation that it would pay off in this election, but that it would encourage more (and more mainstream) third-party candidates to run in the future.