There seems to be a lot of confusion in regards to the modern experience in SharePoint versus the classic experience. One of the main ideas behind modern is to create an out of the box modern, responsive SharePoint site that doesn’t look like the traditional out of the box classic sites. Many business units in organizations frown upon the classic site due to it not being visually compelling.
With classic sites, we can customize the master page or add custom CSS to create a visually compelling page. The issue with this is that it takes a good amount of work for a customized site to then be made into a responsive site.
One of the main advantages with modern is that it is natively responsive, although your customization options are limited. You shouldn’t modify the master page, for example.
You can use the SPFX framework along with the office UI fabric to customize a modern site.
The disadvantage of modern is that not all the web parts you know and love have a corresponding web part in modern. For example, task lists and search display templates can’t be used in modern.
You might say: “Why should I move to modern?”
One of the main reasons is that Microsoft is adding all new functionality to modern and not classic.