Tier 1: Need to do basic attacks on enemy heroes for maximum healing efficency.
Tier 2: Using Divine Star on enemy heroes makes your heal better and you don't have to make manual basic attacks.
Tier 3: You don't have to use flash heal on yourself that much because it heals you when you heal your ally.
Tier 4/Ult.: Heal, and protection (can be interrupted)
Tier 5: Using Divine Star makes you run faster, you can bend the returning heal wave more easy, potentially healing more ally.
Tier 6: You can use Flash Heal more often if used on different heroes.
Tier 7: Ult. heal more, give movement speed to allied heroes, making it better for escaping or reversing a failng teamfigt and pursuing the enemy afterwards.