Why is Williams taking outside capital at all?

Williams, in an interview earlier this week, cited a few reasons: As Medium scales, taking money from multiple investors is a signal of long-term thinking and diversification to the company’s employees; and the more parties that have a stake in Medium outside of Williams, the more they have a stake in the company’s success.

Williams also specifically picked Sze and Elman for board seats for different reasons. Sze has a good investment track record, having sat as an observer on Facebook’s board of directors, and he is a current director on LinkedIn’s board. Elman and Williams go back to their days working together at Twitter, where Elman was a product manager on the company’s growth team.

And lastly, Williams can tap into the networks that outside investors bring with them — often something he doesn’t have time for while working on product and running his company full time.

TL;DR: As a signal to employees, to get Elman + Sze, and to tap into networks.

Kara Swisher and Mike Isaac report that "Medium is about to become Large":

Among the multiple parties involved in the round are Google Ventures (courtesy of general partner Kevin Rose), SV Angel’s Ron Conway and a number of other investors, such as Chris Sacca and Peter Chernin.

And more: Tim O’Reilly, Michael Ovitz, Gary Vaynerchuk, Betaworks, Code Advisors, CAA Ventures and Science.

The single largest contribution to the round, however, comes from Greylock Partners, the Silicon Valley firm which has previously invested in companies like Facebook, LinkedIn and Tumblr. So large, in fact, that general partners David Sze and Josh Elman will join CEO Evan Williams on Medium’s board.

That's a lot of constituents.

Medium does pay some of its writers:

Though Williams has long said the company is a “new place on the Internet where people share ideas and stories that are longer than 140 characters and not just for friends,” outsiders still haven’t been able to grasp just exactly what that means. Moreover, due to its early invite-only status and high-minded design principles, many viewed it as a space for high-quality content only.

Williams maintains that’s not true. He’s said he has taken a come-one, come-all approach to content on the platform, not barring any one particular form of content over another. “We welcome all the so-called ‘crap’ as well as the longer essays. But Medium isn’t just a long-form platform,” he said in an interview.

Ideally, Williams envisions Medium much like a magazine creative director, inviting the types of items that may show up in a magazine, from features to top-ten lists to cartoons to even video. Williams has also taken to hiring editors for different sections of Medium, though a Medium “editor” isn’t like a traditional magazine editor; section editors essentially work in talent discovery and story development, finding talented writers, inviting them to the platform and working on ideas with them to fully flesh out their stories.

Medium pays some of its writers, but more to spur the network’s creativity and invite others — who aren’t necessarily professional writers — to use the platform as an arena for self expression.

So...

Medium = Blogger x BuzzFeed ?

Let Lil Kim break it down for you a lot more simply: "Why spend mine when I can spend yours?"