Its entirety contextual. A framework will let you protoype site very quickly. It will also allow you to bang out sites [that will look halfway decent] quickly. They provide styling for things that are a pig to style manually (form elements in particular).
From a learning perspective, not understanding the underlying language makes it difficult to debug when issues arise. The framework becomes a series of magical incantations one invokes to make things happen [
round about the container-fluid go, in to breakpoint'd rows throw, col-xs and gutters none, &c] rather than a certain way of structuring code in a given language to fulfil a common need. On the flip side, understanding how a framework does what it does by actually reading and understanding [good] framework code, that’s a good use of time.
Ex 1: agencies tend to like Bootstrap - their business model is predicated on banging out sites that look good in as short a time period as possible. They need to do that to survive financially. Jamming together a pile of libraries and frameworks is much faster than carefully writing everything by hand.
Ex 2: A successful business that is dependent financially on one application (say an online store) is more likely to want to have a high level of control over all aspects of the design - in that case Bootstrap, though useful at first, starts to get in the way quite quickly.
Ex 3: A government body is likely to have stringent rules surrounding design and accessibility; Bootstrap is not a good fit, because the sites need to be built exactly to specification; it makes more sense to actually convert the spec to HTML structure/styling, to write a framework instead of using an off-the-shelf product.
Note that sites on awwwards et al mostly fall into one of three categories:
- Agency projects [generally adverts] designed to show off the agency.
- Personal portfolios of people who want jobs at agencies, and have generally hand coded something nice for their CV [again, adverts].
- Tech teams [at e.g. Google] who are showing off skills [again, generally adverts].
The agency sites in particular are often explicitly designed to win advertising awards. They’re not necessarily best practice, or useful, or anything more than very, very pretty; whether they use frameworks or not generally has no bearing.