Hmmmm I started using it for the modulation matrix controller, but there were things I wanted to do that I couldn't, so I wrote my own script panel matrix controller using LAF instead.
LAF is - I guess - a subset of the JUCE graphics API, so I'm more comfortable with LAF. CSS hasn't really fully clicked for me yet, if I'm honest. I've never needed it for anything else I've ever done, because none of it was web based and it was all Python, c++, JUCE, Lua, and C#.