Mana Shield is cheap method of protecting your Hero, tanking a small amount of health loss. Compared to life-healing cards (e.g. Quick Break), shields such as Mana Shield are independent from lives and have no limit.
Compared to buying more Bloons to attack the opponent, some players may bring Mana Shield to offset some of their own hero damage during the early-game.
Mana Shield is easy to get out of your hand and heals a substantial amount for its low cost, making it solid for non-Obyn OTK or Control decks.
Using 3× Mana Shields, any hero in the game can get 240 extra effective max health.