It's easily one of the best cards for holding board advantage.
Let's assume you are looking at it using the common arena assessment; you get the card without anything else on your side of the board, without any other specific cards (so no certain combos or follow up), on any turn you could play it (but with emphasis on the first one).
The main scenario where it's strong is, well, that. You get a Harvest Golem to play onto your empty board at the start of turn three (or coin into on turn 2) and it's more likely you will get a body left on the board going into turn 4 than with any other 3 health minion or 3-drop in general. It is also good when playing against late game AOE to have something left on the board that requires some extra resources beyond the first spell. Against turn 4 consecrates or turn 7 flamestrikes that can be a significant advantage.
Looking at all neutral common 3 drops this becomes even more obvious.
The Jungle Panther's stealth means it will likely survive the turn, but then it's only likely to trade 1-1 and any 2 attack 1-3-drop will be able to clean it up. A halfway awake opponent can play around it and deny it the option of trading up (unlike a charge minion) or hit it with AOE (plays right into a turn 4 or 5 consecrate). It actually gets better late game, assuming they don't have that AOE to waste on it, when it's more likely to trade up so long as you can survive the turn and they can't drop a taunt in its way.
Razorfen Hunter gives you an extra body straight up, but only at a 1/1 (unlikely to trade up, likely to be pinged, potentially lets them enrage a minion) and more vulnerable to AOE cleaning both of them off the board.
Scarlet Crusader is really nice, but if they clear her on their turn the odds are they can do it more efficiently than clearing a Golem. Any two 1 damage hits kill her, compared to a 3 and a 1 for the Golem, so unless playing onto a board where they will have to trade down to deal with her you would get better value from the Golem.
Shattered Sun Cleric comes close to being more valuable, but needs to have another minion on the board to be effective. It's painful to play her out as a 3/2 3-drop if you aren't getting that buff. Ironically running her alongside the Golem means you are more likely to always have a buff target.
As for why it ranks so highly overall, 3-drops are generally valuable. Given the way most decks build their mana curve you would expect people to value 3-4-drops most highly. Looking at
Trump's rating you see the two most powerful common neutral 4-drops, then the two most powerful common neutral 3-drops. I'd almost reverse this, as there are lots of excellent class specific 4 drops and only a handful of such 3-drops, although that does obviously depend on the class you are playing.