In General, to better understand what you need Bootstrap for, you can go back a bit and answer the question “ » What is a css framework anyway?”
In fact, in simple terms, it is a file or several files with ready-written code that are connected to the site in the head section, after which it becomes possible to use the features of this framework.
Frameworks are created to make it easier for other web developers to make up sites. I already mentioned at the beginning that today almost any developer after creating a couple of sites from scratch is thinking about how to speed up the development process.
The fact is that if we are going to do site development from scratch, we will have to take care of a lot of things. All css styles, all web scripts will have to be written from scratch, and it can be hundreds or thousands of lines of code. Moreover, you can make a lot of mistakes in the layout. For example, your template will simply look different in the main browsers, or it will not be adaptive.
In General, just for the sake of adaptive layout, it is worth using Bootstrap, because if we are talking about fixed layouts, they are easy to make even from scratch. Just create blocks, set them a fixed width, and work on the layout.
But in the case of adaptive layout, everything is much more complicated. You will need to make sure that your site displays well on all screen resolutions. To do this, you will need to use media queries. For large templates such queries may take a very long, besides, you still have to learn how to write them.
In General, when developing an adaptive template from scratch, you will have to work hard, and your skills in layout should be quite high.
What about bootstrap? If you study this framework, it will greatly simplify the layout for you. First, the framework takes on cross-browser compatibility and adaptability, and these are the main things that the developer should take care of. But with bootstrap, it’s very easy to implement them. This allows you to create an html template even for a person who has previously done very little layout and is not particularly familiar with css.