There's quite a lot of legit uses still. And also, it's not about speed, if I'm on a hotel wifi I don't want firefox giving up if I have multiple files queued up from my file server. Sure, browsers support pause/resume but they also fail pretty hard on spotty connections. With jdownloader you just set it and it'll do it's thing.

Also nice if you're manually downloading game mods, some are hosted on file lockers that make you wait so many minutes. Again, jdownloader was built for that.

I also say that as someone that really can't stand jdownloader, but the alternatives are... not great. Surge, the tui, is nice if you're just doing regular ol' http downloads.