2how to use it
The buff comes from interacting with the placed station rather than from carrying it, but nothing stops you from mining it straight back up. Break it, pocket it, and the armor penetration stays with you, so in practice this is a portable buff you set down for one second before a fight. All it needs is a small clear space, which is the only real constraint underground.
Armor penetration subtracts flat defense from whatever you hit, so its value swings hard with the target. Against armored enemies with modest health it is a big proportional gain. Against a boss with an enormous health pool and low defense it barely registers. Events packed with tough trash are where it earns the most.
who it helps
Melee only now, both the weapons and the projectiles they throw. A summoner or ranger gets nothing here. The buff holds until you die or leave the world, so top it up after every respawn, and if you never found the Jungle cabin version the Merchant will sell you one once Hardmode starts.