Observing enemy leaders can offer a bonus stance point on top of the one gained by defeating said boss, so it's worth learning how to do it.
Ghost of Tsushima: How To Observe Enemy Leaders
When you get into Mongol territory, you'll need to stay undetected to be able to observe the leader.
They'll be wearing shiny gold or silver armour, and are usually practising their combat skills somewhere within the encampment. Once Jin gets close enough, follow the prompt and press R2 to observe the leader.
This awards a stance point, and you can earn a second by killing the boss.