If you have an ability that triggers when you do damage to an enemy, pierce gets it to go off automatically.
Not necessarily. The ONLY think Pierce 1 does is negate one block. It by no means guarantees that damage will get through.
Peirce 1 is inferior to +1 Damage if there are no blocks that need piercing and equal to it if there are. There is no situation in which Peirce 1 would have benefit over +1 Damage.