Actually, if I tell you in the simplest way then frameworks make your life easier and you don't need to build everything from scratch. You will get models to work with databases, Controllers to write business logic and views to show the frontend. Everything built feature will be aligned and remain separate from others in a nice way.
Well, it really depends on the size of your project. A homepage with no database and no more than 10 pages, a framework is going to be too much, mainly because frameworks always need a lot of configuration and customization in most cases, to fit specific needs. And in some situations, they could be a little slower than several pages included in a file (think about all the framework needs to load to use its features.).
Now, if you are planning a mid/big size site, with database interactions, web services, etc etc, you will need a framework to help you interact with different technologies and organize your code in a way that when things break, you'll have the ability to quickly detect and repair it. Also if you think about clients, they are always changing their mind about software, so if the client needs to modify or add a new feature you should not have to go through all your code and think about how do I plug this new feature in all this spaghetti.