People don't use bootstrap just for the grid, there are N different CSS frameworks to implement grids. What bootstrap brings to the table is a predefined and standard set of CSS widget styles (buttons, text-boxes, drop-downs and most importantly, the navbar). There are very few other CSS libraries which provide such widget styles and bootstrap gives it away for free through the MIT license! This is the reason for bootstrap's rapidly increasing popularity.
As a back-end webdev who doesn't know a thing about front-end styling, I can take the bootstrap theme and get working on my app since day one! There are no other CSS frameworks that do this as perfectly as bootstrap does. There are others like zurb foundation and bulma but they are light years behind bootstrap in terms of stability and comprehensiveness.
Using Bootstrap UI components does not require you to use Bootstrap for layout.
The crazy thing to me is at some point you did learn how the Bootstrap grid works, and I assume you learn how new tools work as a part of your job all the time. But with html and css folks who describe themselves as backend seem to be like "nah".
When bootstrap first arrived there was only floats for grids & nobody was doing responsive design. The first bootstraps did not include responsive elements. Fontawesome wasn't included (gyph). Designers started using it mostly for the JS parts (carousel etc) & grid math.
Bootstrap is great to make an instantly nice site & is useful.
Now that grids/variables etc are here, it is not worth it but I can see why people like it (deep stuff like btn state is done).
I use Bootstrap just for grid ;)
CSS Grid is great, but hard to work with a team of junior devs
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