It's probably a mixture of potential impact (Medium posts will always be much more popular and easier to find on Google than some static page he could set up) a

It's probably a mixture of potential impact (Medium posts will always be much more popular and easier to find on Google than some static page he could set up) and ease of publishing (just write the article and publish it, versus managing your own blog platform), plus some added bonuses (sharing widgets, analytics, etc.).