Benefits? What benefits? Bloated markup, violating the separation of concerns, flipping the bird at usability, accessibility, and the very reason CSS is separate from HTML in the first blasted place?
Every single flipping claim of them being "easier", or "better for collaboration", or "making you more productive" is a bald faced lie. One parroted by people who never learned enough HTML or CSS in the first place to have a valid opinion on the topic,
Thich makes sense since it's clear the bunko peddling fools who CREATED all these frameworks are unqualified to write a single blasted line of HTML or CSS, much less have the gall to tell othes how to do so!
ANYONE telling you there are benefits -- ANY BENEFITS -- to using these frameworks is talking out their arse, packing your fudge, and full of more manure than Biff Tannen's 48 Ford Super De Luxe.