Like most Magic: The Gathering cards, the value of Circle of Protection: Green depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (30th Anniversary Edition)
Decks playing at flash speed like Heliod, the Radiant Dawn can make the activated ability enough of a barrier that people wont attack you without ever even needing to activate it
Even if Auto Tap is set to On, you can override it at any time in the game by entering Full Control (which you do by pressing Ctrl on desktop, or on mobile by tapping and holding your player avatar to enable or disable full control)
Its definitely a lifegain card in 1v1 Magic where you usually start with 20 life
Larry's Dudunsparce ex
The Wild Pikachu Card Fled: The Importance of Retreating Energy also plays an important role in the game in terms of paying for another mechanic, retreat