Pennywise and Mary Poppins are just two avatars of the same larger, trans-dimensional, intelligence. By using human archetypes of the “Seven Deadly Sins,” it is giving us the tiniest glimpse of itself in a way our feeble human brains can understand. Picture a seven tentacled octopus the size of an entire galaxy sticking the tip of a single tentacle through a hole in a fence.
Pennywise shows up in Derry - drawn by the children - every 27 years to feed on fear. The embodiment of the deadly sin of gluttony. Water-based evil spirit. Fond of paper boats.
Mary Poppins turns up at the same family’s house - drawn by their children - every 54 years (27 x 2!) to feed on ingratiating compliments. The embodiment of the deadly sin of vanity. Air-based evil spirit. Fond of paper kites.
In addition to feeding on their fear, Pennywise steals children unless he is stopped. Stealing the children is “Mary Poppins’” real endgame - and she’d manage to get away with it, were it not for Bert.
Bert functions as some sort of Eternal Protector - ageless and standing guard, only to reappear when Mary Poppins once again threatens our realm. The “Champion Aeternum.” He placates the beast with relentless compliments and empty flirtations until she is sated. She then flies back to her secret lair.
Bert and Mary are bound together - doomed to battle throughout time as long as either the Banks family or Mary exists. Bert is a space turtle.
Logically, we must conclude that there are 5 others Avatars (Greed, Sloth, Envy, Wrath, & Lust) that we have yet to encounter - or if they have already visited us, we have yet to recognize.
Vigilance is key, people.