LESS is a cascading style sheet preprocessor; we can think of it as a server-side script for CSS files, enabling us to do a number of things that CSS doesn't support (yet), just like PHP and/or ASP can do for an HTML page. The following diagram should help us better understand the concept:

These are the main advantages of using a hypertext preprocessor instead of writing raw HTML pages; we're talking about PHP, but the same goes for ASP.NET Web Forms, Razor, and basically everything else.

The following are the advantages of using LESS instead of writing raw CSS files:

As we can see, they serve the exact same purpose in terms of assisting, improving, and enhancing the development effort.

Making the switch from static style sheets to dynamic style sheets is just as easy as switching from static HTML pages to PHP or ASP dynamic pages; they both feature a nested metalanguage that can extend the base static language in a pure backward-compatible fashion. This means that a valid CSS file is also a valid LESS file, just as a valid HTML file is also a valid PHP or ASP file.

There are also some key differences between hypertext preprocessors and style sheet preprocessors, the most important being how web servers deal with them. Hypertext preprocessors such as PHP and ASP are compiled by the web server upon each request; the web server compiles them on the fly and then serves the resulting HTML for each request-response flow. Conversely, style sheet preprocessor files are usually compiled into standard CSS files before being published; in other words, the web service doesn't know about the existence of these files, as it just serves the resulting CSS-compiled result.

This also means that using a style sheet preprocessor will have no performance impact on the server, unless we choose to install some experimental and still highly inefficient handlers, extensions, modules, or client-side scripts that will compile the source files on the fly.