One guess that I'd have is that a16z is looking for synergy. That means they're not really thinking of Atrium as an isolated home run investment. Instead, they're looking at them as solving a significant issue for the companies they invest in. If a16z sends a stream of business their way, they'd not only get the $65m, but a lot more in business. And from a16z's perspective, Atrium could solve a major headache/risk factor for early stage companies, so $65m is a small price to pay for all the returns they get from their other investments, even if Atrium never exits.
And if there is an exit, it's the best of both worlds.
I believe that being able to work with Marc Andresseen, Andrew Chen and the A16Z team can significantly positively impact the trajectory of the company!
In addition to what others have said, raising from quality investors is a positive signal to Atrium's potential customer base. It adds credibility and makes growth easier.
Already venture capital unbundles cash from credibilty/endorsement. Angel investors provide both. A VC uses their own credibility to scout good investment opportunities, which then receive cash from limited partners. VCs often invest in their own funds, but just enough to make limited partners believe they have "skin in the game": the real thing they have on the line isn't capital, but their reputation as good deal-pickers, which let them raise funds later from their investors, limited partners.
I wonder if someday, there won't be a class of people who are pure reputational endorsers, who say "this company is going to do well", and that signal by itself is enough to attract a big amount of pure cash.
Maybe the "I like this" and the "I'm putting my money in this" have to be the same in order for others to believe the "endorser" has enough skin in the game.
