Re: Sidechain purpose
Hah, I didn't think you still posted here. Glad you came back.
Honestly, ducking is just a technique, IMO. It's a tool that can sometimes enhance (or destroy) a mix. There's no more magic to it than there is with regular EQing, compressing, dynamic EQing or multiband compressing. They're just tools at your disposal.
With that said, if you're making Goa trance or some shit it's probably standard to SC everything to your kick for that particular pumping sound. TBH I don't like hearing it in metal, but plenty of producers use it there as well.