"the existence of a secondary market contributes to at least some primary sales"

Incidentally, many favorite books that I pirated when I was broke and had very little room, I later bought paper copies of.

I also pirate most music, but if I really like a band I discovered via piracy, I will buy their vinyl or see them in concert.

The ability to freely share content scondhand, be it a paper book or in digital form, creates new fans. Some of those fans have money to give back, or will share with people that do.