Merge stylesheets in a fileeditor
If you activate this option, it can increase the your site speed. Because it reduces the request count and it is able to cache files.
After compiling the dynamic CSS to files, separated files will be created for each page inside the uploads folder. And you can realize your site work at a better speed.
After merged stylesheets(css & js) into a single file, if you update pages or theme options and some various theme & plugin settings, you must recompile css and js.
Please see below image.