Combining - When your website has lots of small JS and CSS files, your website must make individual requests for each file. Each request can add 200ms of time to your page load and puts increased load on your website server. While Internet connections have gotten faster, the delay introduced by requesting lots of small files hasn't.
Minifying - Generating .min files for CSS and JS has long been a common way of reducing file size, but the space saving is nothing compared to .gz compression. Minify generates .min files for use when a Web browser does not support .gz files. All modern--and even older--browsers support .gz files.
GZ Compression - Pre-generating compressed versions of your static HTML, CSS, and JS files will significantly reduce the size of your files. Where a .min may be 80% the size of the original, a .gz may be 10% or even less. The size reduction makes the difference where your page will load with a snap and makes an even bigger difference when being loaded over wireless Internet. Some web-servers will allow your website to automatically generate GZ compressed pages, however this puts increased load on the Web server and pregenerating the .gz files ensures as little load as possible.