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 ...