Why does tank stance exist? So that newbie tanks and people doing blind prog can put their "training wheels" on. This makes the role that people are already afraid of easier to get into and lets you learn fresh content with less wiping(because let's be real, no death screws up the raid as a tank death, especially when there's forced tank swaps involved).

Why does tank stance need the damage penalty? So that once you learn to plan your mitigation correctly, you can get rewarded for it with more damage - same as healers get rewarded with more damage gcds once they plan out their heal cds. It adds more room to improve your gameplay.