Not sure how bad it is these days, but Adobe Reader used to open pretty slowly (and if you had Adobe Acrobat open your PDFs by default, it was even slower), so an in-browser PDF viewer was appreciated for that purpose.

Also, it can be useful to keep the PDF in the context of the app you opened it in. Unfortunately, I'm not aware of a window manager that surfaces the fact that, for example, one macOS Preview window was opened from the browser, another from Slack, another from Finder, etc. Compare to iOS, where opening a PDF viewer from an app will result in a button at the top-left corner to go back to the app you opened it from.