A CSS Frameworks is a package which contains a set of files and folders with most commonly used elements to support website building process. It provides a basic structure for websites so that the developers doesn't need to design everything from scratch.
Why we need Frameworks?
Without frameworks, there will be no common structure for websites and the designer has to do a lot of work from scratch. The reuse of work is very hard and take a lot of time to design even simple websites. But with the help of frameworks, you can create complex websites within few hours.
Problems in using CSS Frameworks.
CSS Frameworks are a set of inbuilt codes and that???s all, it is not a language to design a website. It is bound to have inconsistency, for instance, a UI element like gallery may not work in web pages with table structure, etc.
If such case is encountered, you should either try using a different element from third party or you should edit the code of the UI element itself to make it work. These are rare cases, so you don't have to worry about it.