You have a full board, and you want to win
Its drawback may look like its very punishing and can allow the opponent to surprise you via a mid-combat spell, but I never really saw that scenario play out since its damage prevention ability is a a conditional static ability so as long as its on when damage is dealt, it wont be prevented. (Thank you, @fenaris!) It usually plays out somewhat like a colorless Ashenmoor Gouger / Chimeric Idol where you typically dont bother trying to block with it (why am I blocking with my aggro guys anyway?) and, if need be, you cast a spell pre-combat to activate it (typically I used a large coin, like a quarter, to denote whether it was on or off)
Hes also got a strong ability that lets you draw two cards whenever hes played, making him well loved by players and collectors alike
Annhilator 1 might not seem like much, but its also a hard counter to graveyard-based loops
Space Marine Devastator Our penultimate(ish) charting card is this