The reason: it is very user friendly, the interface is beautiful and, even for more experienced people, it brings a little more confidence than using dd directly.
I wouldn't call it a piece of thrash.
That being said, I'd much rather not install a miniature Chrome browser just to provide the UI for a command line tool, so I'm glad to see non-Electron as a selling point.
Edit:noticed this is sort of a repeat answer, leaving it so the people saying its slow can copy/paste and try it out.
I understand the appeal of Etcher, but it seems like such overkill to launch Chrome just to write some data to a disk. I wish Windows/MacOS shipped with good tools for this out of the box instead. Disk Utility seems to be getting less useful over time though.
I'm FULLY aware that this premise has enabled the vast majority of garbage software that exists, but I think perhaps it's good to remember that we can have software that is both "very very mindlessly good" and "absolutely also respects users," Syncthing comes to mind here.