They started as a Detroit-only grassroots effort called "Why don't we own this?", tracking property auctions and helping locals buy adjacent parcels when they came available.
But quickly they realized this data was powerful once it was easy to access. Which was no mean feat -- every locality has their own terrible way to store parcel data, it seems, and it's a ton of work to gather and unify it. But once they've done that work, they can charge a bit to access it in scriptable and exportable ways, but the browser interface remains free.
Personally I think the government should just fund the operation and make all the access free, because this is fundamentally everyone's data, and unifying it should be the government's job. But in the meantime, I've never found myself needing more than the free access.
