The attack gained from destroying a monster is permanent.
It gradually gains attack.
It can prevent monster effects that activates upon getting destroyed by battle and then sent to the Graveyard.
Initial attack and defense is pretty low which makes it an easy target.
Having a low initial attack makes it hard to destroy a monster in battle in order to increase its attack.
Most level 4 monsters have higher than 1500 attack, making it difficult for Trainee to be able to destroy anything, but there are a lot of monsters with lower than 100 defense. Use position switching cards like Security Orb, or Enemy Controller to protect your monster so Trainee can destroy them without activating their "sent to graveyard" effects.
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I don't get the hate for this card tho. This card is great because it can be a threat for those who runs Blue Dragon Summoner (Last Gamble decks), Yomi Ship, Gravekeeper's Recruiter (need a booster like A.Forces to overpower the 1500 def.). I know that the stats are not great either.