Flask is a server-side framework, so it’s not directly comparable to front-end frameworks like React or Angular. It plays a similar role to Rails in Ruby, Django in Python or Symfony in PHP, though it’s further towards the lightweight/flexible end of the spectrum than any of those.

I don’t know what you’d consider high traffic, but we have used and continue to use Flask in production for a variety of systems. Those include:

- providing the back ends for relatively large and complicated single-page applications;

- implementing more traditional web sites, with very little JS on the front end, which use Flask to do almost all of the routing and rendering; and

- implementing dedicated HTTP-based APIs for integration with third party services.

Flask is an excellent tool, and that is not a description I use for very much in the software world. It provides widely useful functionality, it has a tidy design and a reliable implementation, it’s well documented, and it imposes few overheads or unnecessary dependencies. If you’re purely a front-end dev then that functionality isn’t what you need and neither is Flask. If you’re interested in the back end as well, you could do a lot worse than to learn Flask.