Compared to Bloon Embiggen's health boost, Pack Protection is one Gold cheaper, but gives 40 shield instead of 25 HP. It's less useful for rushes, but still good for general defense for your bloons, even without the resulting damage you'd get from Bloon Embiggen.
Pack Protection may be helpful later in early rounds when sending large amounts of bloons, so it's harder to break rushes, especially if the opponent cannot afford a Thunder Druid or Super Monkey Storm on the turn after yours.
Pack Protection after bloons become On Fire also prevents this effect from damaging any friendly bloon, as it only damages the shield. Likewise applies to Acid Pools.