a lot of academics aren't super technical and don't want to deal with git workflows or syncing local environments. they just want to write their fuckin' paper (WTFP).
overleaf lets the whole research team work together without anyone needing to learn version control or debug their local texlive installation.
also nice for quick edits from any machine without setting anything up. the "just install it locally" advice assumes everyones comfortable with that, but plenty of researchers treat computers as appliances lol.
The visual editor in Overleaf isn't true WYSIWIG, but it's close enough. It feels like working in a word processor, not in a code editor. And the interface overall feels simple and modern.
(And that's just for solo usage -- it's really the collaborative stuff that turns into a game-changer.)
Overleaf ensures that everyone looks at the same version of the document and processes the document with the same set of packages and options.
Then: The LaTeX distribution is always up-to-date; you can run it on limited resources; it has an endless supply of conference and journal templates (so you don't have to scavenge them yourself off a random conference/publisher website); Git backend means a) you can work offline and b) version control comes in for free. These just off the top of my head.
You can even export ZIP files if you like (for any cloud service, it's not a bad idea to clone your repo once in a while to avoid begin stuck in case of unlikely downtime).
I have both a hosted instance (thanks to Overleaf/ShareLaTeX Ltd.) and I'm also paying user for the pro group license (>500€/year) for my research team. It's great - esp. for smaller research teams - to have the maintenance outsourced to a commercial provider.
On a good day, I'd spend 40% in Overleaf, 10% in Sublime/Emacs, 20% in Email and 10% in Google Scholar/Semantics Scholar and 10% in EasyChair/OpenReview, the rest in meetings.