What does it provide that is so special?

Its provides detailed analytics on the popularity of different websites and the keywords that people search for; it goes really, really deep on this, much deeper than you can go through manual google searches and various scripts. You can easily recreate Vercel on your own if you're willing to forgo some convenience, for example; there's no equivalent to that for Ahrefs - you can't realistically 'roll your own' or code your own Ahrefs. The web largely works on an advertising model, and Ahrefs is what tells you what has value for advertising purposes and what people will visit, or are visiting now. If you're doing something simple, it may not matter. If you're not, Ahrefs is indispensable.

I'm not sure I buy that. Surely ahrefs is depending on the data that Google provides and if that is the case, there can be a competitor.