Heart openers are a large part of a well rounded yoga practice. They offer many benefits, both physical and beyond. But opening the heart space is much more than stretching the chest.
Physical benefits of heart openers include improving spinal mobility and posture, improving lung capacity, relieving upper back and neck tightness, and stimulating the thyroid, pineal, and pituitary glands. Beyond the physical, chest and heart openers focus on the heart (or anahata chakra). Anahata translates to "unstuck", and symbolizes the return of the heart to it's pure, full, intact nature (prior to being damaged by anyone or anything). Heart opening postures are said to improve compassion and connection to others, as well as alleviating symptoms of depression.
Chest opening postures can be done via backbending, binding, or via restorative postures using props. It's important to understand that the modern yogi's postures typically includes a rounded upper back, rounded and internally rotated shoulder girdle, and closed chest position (due to sitting, computer use, driving, etc.). Therefore heart openers are big postures and must be appropriately progressed.
The lengthening and opening of chest musculature (the pectoralis major and minor), external rotation of the shoulders, and retraction and depression of the scapula all must work together to provide maximal heart opening. Therefore it is more than just chest stretching. You must include strengthening of muscles such as the rhomboids, lower trapezius, and external rotators of the shoulder. This balancing of strength will assist in keeping the open chest posture long after the yoga class has finished.