It keeps your hand full and your land drops consistent, especially against ramp-heavy tables
Thalia, Guardian of Thraben Thalia, Guardian of Thraben often fulfills the taxes half of Death and Taxes, a strategy utilizing disruptive creatures to knock their opponents off-balance and close the game before they stabilize
Perhaps the motivation to create the subtype is strictly flavor-driven, with unclear plans on how this could be activated via rules in the future
These decks would be my starting point
9: You get an emblem with "Whenever a player casts a spell, you create a 2/1 blue Ninja creature token." +1: Up to one target creature you control can't be blocked this turn
Death is only an inconvenience with Olivia, so let's also make use of both Ashnod's Altar and Phyrexian Altar to sacrifice creatures and make mana at the same time