In a scene where your character gets angry, you want the reader to share that anger. If the character is scared, you want the reader to feel that fear. If a character being interviewed for a job feels nervous and his leg is bouncing up and down, it’s very rewarding to be told by a reader that their knee started sympathy-bouncing when they were reading that scene.
Putting the reader into a character's shoes by having them experience the same emotion is a powerful tool and a great way to form a connection between story and audience. But this isn’t always just a simple matter of describing what the character is experiencing and hoping the reader will be immersed into their world.
Moreover, not all states of mind are equally interesting to be immersed in. An insane character, a disoriented character, a bored character, these can all be accurately conveyed, but should they be?