Coding in raw CSS can become tedious if you do it the old fashion way!
With LESS and the less compiler tool you can now take advantage of variables, functions, operations, Mixins (which is like macros in spreadsheet or substitution)
Look at the Bootstrap by Twitter, if you look at the bootstrap.css it is very complex and confusing! But when you look at the source code of Bootstrap which is written in LESS you will see it makes a lot of sense.
Then use LESS compiler to compile LESS code into CSS code and the output of the CSS is to be used on your site.
When you look at the CSS code you will see how much saving and especially if you also compile in min format.