system
A chat on subreddit /r/pcmasterrace
human
Genuine answer here. Dedicated pdf readers, (specifically acrobat) is able to do many things compared to the PDF reader provided in browsers. I'm sure many office workers will attest to how much they use Acrobat.

Combine PDF files/split PDF files on a large scale. Prepare forms. A big one is digital signatures, which can use Microsoft's certificate authority system.

Basically, you don't need it unless you're in a professional environment.
gpt
Adobe has different forms of the software. I use Adobe Acrobat at work every day, but Adobe Reader is a different program altogether. Afaik, the only thing that Reader does differently from browser viewers is signatures.