Combat math and evasive body large enough to swing in and control a board or gain enough lifelink to offset the crackback
Card advantage or otherwise value engine that keeps asserting a dominant board state beyond just being a big dumb beater
Protection from removal you can use while tapped out or at low cost to be able to tap out for it and not just die 1:1 to a murder
Many have two of those factors. But how many have had all three? Chromium is a scary 7/7 flier that's hard to deal with, but if someone can just rush around him he's not going to develop more card advantage or gain life for you. Prognostic Sphinx can keep generating some value (not CA though) and protect itself, but its a 7 turn clock.
This poops card advantage, protects itself with no mana cost after you tap out for it, can swing big enough in the air to both close out a game and make that big life total swing like baneslayer angel, with the upside that it also might jump up to ~7 power when you add in draw spells.
Lochmere serpent does all of them and then some more utility.