ACF provides WP devs with a simple interface to build custom meta boxes and custom fields w/ a set of extended custom rules.
It allows me to quickly develop a professional and intuitive administration UI and simply retrieve custom fields values in the front-end.
There's a bunch of goodies built-in like the Photo gallery or the repeater field which you almost need/use daily in your WP theme development.
And the best is that you can built your own custom fields –add-ons– on the top of ACF w/ the help of its smart API.
Simple, efficient, affordable.