Meta Box's custom fields have a default padding, color, border, font, etc. which may not fit your design in either back end or front end. So you may want to change how they display, making them more eye-catching. That's the moment you want to style them.
So do the custom fields created by Meta Box. You definitely can style every custom field created by Meta Box as you want. In this post, I will give you some way to use CSS to style the fields.