Not only do you get absurd cost reductionespecially considering how big green creatures are these days (look at the 4-mana 6/5 with upside in Colossal Rattlewurm)but also a potent finisher effect with the attack trigger
Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities are triggered abilities and will have "when," "whenever," or "at the beginning of" in their reminder text
Once you put it back into the Active Spot, you will be able to use Megaton Cannon for two consecutive turns as Retreating removes the attack restriction from Megaton Cannon
Unless you are an old school sicko, or a collector, there is no reason for you to splash out that kind of money
As a gimmick to get Pokemon card collectors interested in the set, the Japanese company introduced Gold Star Pokemon cards into the hobby
Six-y Beast This card is from Unglued II (back when it was called I Swear It's Not a Six Beast)