If you can drop the Sentinel on an early turn, most players arent going to be interested in holding their ramp spells in order to pay your tax
In Standard, it plays an important role as a threat in midrange decks
Unlike the Sword of X and Y cycle, it successfully promotes use of the combat step without discouraging interaction
Its easy to draw two or three cards a turn while dealing massive damage, which is exactly where an aggressive deck wants to be
First, it's cheaper - the blue one is the most expensive one
Like those, it was banned to avoid the perceived-barrier-to-entry, but was strong enough to be a candidate for banning even without the optics especially in long games where the card draw yields inevitability over time