Earlier quoted context omitted.
Part of the reason it matters more to web developers is that often our clients and employers do care what we use and if we love a particular tool it needs widespread adoption to be able to use it on real work. I have potential clients asking constantly if I can build their project with WordPress or Rails or Drupal or Symphony or whatever else they think is the best. It would be much better if they just came and asked…
WordPress is a valid question/option for them if they have a large amount of content they have to add using their marketing staff instead of more expensive developer consultants. It also is if they don't have a years-long working relationship with you, because who is going to maintain all of this when you're gone? At least if they have to dump your work and build a new theme, they don't have to worry about figuring o…
We don't give credit to how good some plugins are, either.
In my current web app I've had to add user-defined custom fields to a few entities. It's been a nightmare (Symfony2's form library isn't designed with this in mind) and the user experience is nowhere near as nice as ACF.
And for web apps, the user experience is as important as what it does. I wouldn't want to build this application on WordPress, but having components like ACF already present, so I don't have to reinvent the wheel, is making me seriously consider it for the next project.