I'm just making this up as I go along.

Suppose you are Medium. Your primary goal in order to survive and prosper is to attract as many paying members as possible. That's your only source of income.

If an article gets 100 reads and if all of those readers are devoted readers and paid subscribers, Medium won't make any additional money from them (though happy readers are of course more likely to re-subscribe at the end of their year)..

If an article gets 100 views from people who are not paying subscribers AND if 5 of those people become subscribers as a result of using their free reads, then Medium increases its revenue as a result of your article. This of course is the purpose of the affiliate bonus program (if someone joins and clicks on your request link, you get a percent of the revenue). Other than that bonus, you're probably unlikely to know how many new members your article has produced. But I wouldn't be surprised if Medium tracked that number and used some sort of algorithm to reward articles (and perhaps writers) who attracted new subscribers.

I doubt Medium will publish the click-through rates. But if it did, you would probably still have to know how many of those click-throughs were from non-paying subscribers in order to determine how Medium values your article. When you click on the stats for a given story, you'll see the ratio of internal views vs. external views. These are not reads, of course. There may be some correlation between external views and the payment for a story. Just a suggestion for someone more statistically minded to consider.