When you build your website you want it to look good and therefore, style it!
Setting up the environment can come with some pain and may require learning about many different tooling and setup procedures.
What HTML+ does for you is set everything up for you so you can just start writing CSS using modern Syntax and preprocessors like SASS, LESS and STYLUS.
All this is so you can get to coding sooner while your website benefits from modern CSS that works on most browsers.
node-modules style files
The engine will handle even CSS files imported from some third party module and process them. This means that sass, less, stylus files are all imported and processed as if they are part of the project.
When you build your project, these files are also imported, processed and shipped with your site