Growth Curve

The doubling clause is what turns this from a filler pump spell into a payoff. On a bare creature it reads as a two-mana +2/+2 with an extra step, priced roughly at rate. But the "then double" is written to compound: it adds one counter first, then doubles the total, so it scales with whatever you have already stacked. A creature carrying four counters becomes ten. This is the counters-matter reward built as a multiplier rather than an adder, and multipliers change how a deck sequences: you want to spend this last, after the graft, the proliferate, the modular triggers have piled up, because every counter already present is worth twice as much once it resolves. The main-phase restriction is the price that keeps it from becoming a blowout. Doubling counters at instant speed would let it ambush removal windows and turn a two-power blocker into a lethal one at the worst possible moment; confining it to your own turn means the pump is telegraphed, and the creature you inflate has to survive to combat before it earns anything. Because it hits a single target, it also stays off the top-end of a token-plus-counters go-wide plan. What you are buying is a return on a creature you have already invested in: the more counters it carries when this resolves, the more it gives back, which is exactly the shape a counters deck wants from the spell it casts to close.