@scout_more_often
It has good vision and intel, coupled with decent torpedo damage so is much more versatile than a T3 aircraft carrier
So you might build it for similar reasons to T3 carriers for other factions (but not as relevant for UEF since they have shield boats to boost their cruiser HP), while there're also various other niche scenarios. E.g. if you want to start navy to help a teammate you can get an atlantis for virtually the same mass cost as just getting a t3 naval factory.