It's not in every other forum system (I have had licences for all of the major ones)
It's actually less helpful than you'd think - experience of running forums tells me it only helps in very specific circumstances, and most of those can be better managed in other ways, e.g. frequently it's used to handle group requests, which should be handled with the group request feature. Other cases are people using it to mimic a help desk, for which the help desk mod should be used instead.
It's also a major performance hit, so slowing every single forum down for a feature less than 1% of the forums use is bad for SMF.