Mutagenic Growth

Phyrexian mana turns a pump spell into a free one, and free is the word that matters: paying 2 life instead of a green mana means a +2/+2 that costs no tempo and, critically, can be cast from a deck that runs no green at all. That second clause is the whole reason this spell escaped its color. A combat trick that plays out of mono-blue, mono-red, or a colorless artifact shell is a different animal from a normal trick, because the opponent can never read it off your manabase; the bluff is invisible. The life cost is the lever that pays for the power. Two life to ambush a blocker, push two extra damage through, or win a damage-based fight is a rate aggressive decks happily eat, and the Phyrexian discount only ever helps the deck that can least afford to dip into green for a single card. The downside is structural rather than situational: it does nothing against a board with no creatures, and it gives the opponent a clean read once life totals start mattering late. But for two life and a card it warps a combat step the opponent thought they had solved, and it does so from any color of deck in the game. That portability, not the +2/+2 itself, is what keeps it relevant well beyond the green decks it was nominally printed for.