Hey, I was using LESS for quite some time and switchted now to Sass. For me both are at a pretty same level.
And yeah, I actually can't think of a time before preprocessors. Splitting style components into different files, so you have everything organized. Components, forms, layouts, media stuff, icons and so on.
Variables are a huge advantage, extending existing classes, so you minimize redundant styles and mixins. I mean they just safe the day. You can make complex mixins like svg backgrounds with png fallback and retina support and then just use them like @include svg-background(my-awesome-image);
And there are some "frameworks" with various mixins like lesshat or bourbon for sass.
If you working with task runners like gulp or grunt you setup your file once and you're fine.