Exposure is one reason. Two of my articles have over 3k views. On medium you can make good content and get a decent amount of views much easier than hosting own blog and growth hacking.
Also, a substantial amount of views come from search engine traffic with almost every bit of content on the internet, unless you're doing it wrong. Your most recent post is full of frequently-searched terms. It seems unlikely that little of it is coming from search engines, and almost impossible that most of that viewership is coming from something other than Medium's discovery.
Maybe try A/B testing with your next article? I think we'd all be interested in that.
I just checked it and my most read article has about 170k views. About 50% of that came from HN and direct URL. Only 8% from Medium.
See for yourself: https://imgur.com/BAICxnm