Reason for >8 channels in an audio codec
Since it will most likely not hurt to have the possibility for more streams, this might for one be just to future-proof the system. Nothing would be more stupid than to specify for 8 channels and suddenly 9.1-systems become the next big thing and your codec is not suitable.

Other than that I can imagine bigger scenarios with much more channels such as recording all the voices in an orchestra. But I honestly don't know whether or not AAC is of any importance there.