While the ability doesn't prevent him from being destroyed in battle, his high stats make that a rare occurrence. This makes him irritatingly difficult to remove.
Obelisk excels defensively as well: he cannot be targeted by spells, traps, or monster effects. He can also sacrifice any two of your monsters and forfeit his attack for the turn to basically activate a free Raigeki, wiping your opponent's field of monsters. Obelisk is reliable in battle with fixed stats of 4000 ATK and DEF, making him one of the strongest monsters in the game. Similarly to Slifer, you can special summon him from the graveyard, but only for a single turn.
Like the other Egyptian gods, you need to sacrifice three monsters to summon Kaiba's blue titan, and can only do so in attack mode.