Using a CSS framework for projects is quite common these days whether it be big projects or small side projects but why do dev's use a CSS framework anyway
While making a website, it is a tough job to make headers/navbars with minor feature like disappearing it on mobiles and making a menu instead, handle pads and tablets screen sizes, dropdowns so talking about these we come to our first point
Premade features for easy use and faster development times
though implementing a normal feature like a login page might be easy and quick, adding elements like errors and dialog boxes might be a long and time-consuming task especially dealing with edge cases like overlapping on different screen sizes, messed up animations.
Huge community/organization support for bug fixes, and implementation for many browsers, screen sizes over the years the framework has existed, that means you can get relieved for common problems in older browsers
Help to learn CSS , just getting started with CSS and styling , well it takes a lot of time to learn CSS and proper styling of webpages, you might not know about flex boxes and the new grid layout, well the CSS framework already the easier way to implement it
They come with documentation. If you need help getting started, the framework generally comes with some support files. This can be particularly nice if you are designing a site you will be handing off to a client. You can just let them know what framework you used and refer them to that documentation for support requests.