The value it provides is it simplifies CSS, helping maintainability and preventing repetition. It's not a replacement for CSS, it's just a way to make dealing with CSS easier. The syntax is easier and since it's a superset of CSS, you can keep on using plain old CSS syntax if you want. It adds more options and more flexibility.
Variables alone make LESS/SASS worthwhile, but even putting those aside, let me give you an example of added value: importing other files. With LESS/SASS you can import different files, and they get compiled into one CSS file. With CSS if you use @import, you've got an additional HTTP request, which negatively impacts performance. LESS/SASS gives you additional flexibility in organizing your CSS. In LESS/SASS you can just go to the individual file that deals with typography, as opposed to the tricks designers have used to find things in a large CSS file (e.g. putting a table of contents at the top of the CSS in comments, with a unique string for each section, which you then use Ctrl-F to look for to find the section of rules you're interested in.)
I'd recommend trying it out on a small project if you haven't already, to see for yourself.
Variables alone make LESS/SASS worthwhile, but even putting those aside, let me give you an example of added value: importing other files. With LESS/SASS you can import different files, and they get compiled into one CSS file. With CSS if you use @import, you've got an additional HTTP request, which negatively impacts performance. LESS/SASS gives you additional flexibility in organizing your CSS. In LESS/SASS you can just go to the individual file that deals with typography, as opposed to the tricks designers have used to find things in a large CSS file (e.g. putting a table of contents at the top of the CSS in comments, with a unique string for each section, which you then use Ctrl-F to look for to find the section of rules you're interested in.)
I'd recommend trying it out on a small project if you haven't already, to see for yourself.