It's not really different from Less.app and just like Less.app you don't need it when developing locally using the client-side approach. If you make compilation part of your deploy you basically don't need these apps at all.
Its seems to have a pretty major difference to less.app in that it supports multiple platforms.
Regarding making compilation part of your deployment, you expose yourself to differences between the development and production code which might prove difficult to trace down the line. Keeping your development code as close to your production deployment seems smart to me.
I haven't used Less, although I have been considering it recently.
My point was much more general though. A tool like this seems great to help catch problems early - earlier than your deployment step. Can you be absolutely sure that there aren't any latent bugs in Less or differences between lessc and less.js, such that you could never have any differences between your less.js-backed dev code and your lessc-backed deployed code? Even if the codepaths are identical between lessc and less.js, their execution environments aren't.
All I'm saying is that if you're going to use some _thing_ in your deployment scenarios, it would make sense to use the exact same thing throughout your development (where possible). The biggest argument against this in the case of Less is the hassle of compiling it. That's where a tool like SimpLESS or less.app steps up.
