As the saying goes, if a product is free then you are the product. In other words, if you can't sell your users, you have to charge them.
Users can be valuable without offering net revenue on their own, this is how Github stays afloat, it's at the core of their business model.
In their case, it's like "Free for when you'r working on your own personal passion stuff, but it's paid when you go into the office and hack on your companies private code". The huge community and discussion is essentially a massive funnel, users already have an account, etc.
There's actually some potential parallels with Trak.io and I'm definitely going to keep Github on my whiteboard as a reminder why Freemium can work if you're a tool aimed at developers