My understanding is you can perceive when music looks/feels/smells right or whatever your sensory blend is, which means you can perceive things that work well together that others might not. Of course I could be completely wrong about this.
"I certainly don’t think in words. I’m not sure that anyone does. Does anyone really think in sentences, like in films when you see someone thinking and you hear a voice-over? I don’t anyway, I see stuff visually, as shapes, colours, textures and mechanisms sort of hanging there in space. If there’s a hole in someone’s argument I visually see a hole in it, in the armature and mass of the thing. I’ll see the shape of a whole book that way before it’s written, and so far, the books have all ended up the way I saw them originally."
As for drug-induced synesthesia, the idea would be the same. It can give you ideas, but most of them are likely to only make sense in your own world. It means that unless you really have too much experience with your drugs, the bulk of the work has to be done on a clear head, with a solid musical foundation.