Originally Posted by lustxseo

Subdomains are treated as separate sites, so the website as a whole won't rank for more keywords, only the subdomain will. A link from the subdomain to the main domain won't benefit as much in terms of link juice (and vice versa), the value is severely diminished despite subdomains being treated as separate sites, which is understandable to a degree. Google doesn't want to encourage people piggybacking off one site, and despite that, subdomains are unappealing to users. The only exception would be a highly valuable 1-word domain, but even then, you're better off creating a blog in the subdirectory.
Also, there are no penalties for duplicate content, that's a myth. Of course, you don't want to have duplicate content on your website either way for a bunch of other reasons. If you're creating a blog, then it has to be original.

PBNs are beneficial because they're quite literally different sites, and if, let's say, you're running an anal sex tube as your main bread and butter, then you're creating multiple blog sites for certain anal sex categories (ebony anal, anal gangbang, teen anal, etc.) and writing blogs around those subjects. The thing is, if you treat them with care, update them frequently, then not only will you pass great link juice and potential traffic, but you can also use those blogs for other benefits.
Bottom line being that there is absolutely, 100%, no advantage to creating a blog in the subdomain, no matter what your intent behind creating one is.