Remember that for every reduction in gain(db) digitally, you lose resolution.
With 24 bit, you can reduce 18 db of gain and have lost nothing in detail that even a 1 db gain drop at 16 bit would have. So 24 bit for processing and mixing. 16 bit will suffice for the final master for playback only, which is why there is usually some limiter compression done just before this final conversion. This naturally includes sample playback, but most samplers also process and reduce gain, so there would be loss of minute details.
"Lots of people are nostalgic for analogue. I suspect they're people who never had to work with it." - Brian Eno