Posture is complicated, because it involves habit, training, psychological attitude, and muscle strength. Further, good posture makes you more presentable and makes you look more confident even if you do not feel that way.
Medically, good posture helps prevent back problems.
Computer monitors, age, shame and shyness, muscle atrophy, and general slovenliness are the enemies of good posture: shoulders back, head up, no slouch, belly tucked in, etc. Look good, not proudful but self-respecting.
5 Most Common Posture Problems (and how to fix them)
Prevent back pain with good posture
10 Ways to Have Great Posture as You Age - Here are 10 tips to keep you standing tall at any age.
Posture Power: How To Correct Your Body's Alignment