Burst of Strength

The untap clause is the whole reason to reach for this over a flat pump spell. A combat trick that grows a creature is ordinary green design; the untap is what turns it into something else, letting an attacker swing and then stand back as a blocker, or letting a tapped creature ambush an attacker that thought the lane was clear. The +1/+1 counter, not a temporary boost, means the size sticks after the turn ends, so the single mana buys a permanent body upgrade plus a tempo swing rather than a one-shot trick. The untap also reads as a costing signal: pair it with a creature whose tap is its whole job (a mana dork, a tapper, a sizable attacker you want available on defense) and you double-dip, attacking or activating and still keeping the body up. None of this is loud, and the rate is exactly what a single green mana should buy at common. What lifts it past a pump spell is the second clause doing two unrelated jobs at once: blowing out combat math and freeing a tapped creature, on the same spell, at instant speed, for one mana.