As noted $0 -> $3 is pretty huge. You have a lot of villages at $3, a few draw cards (e.g. Oracle), and a few nice attacks (Swindler, Fortune teller, etc.). While Altar can manage this, Altar cannot go $5 to province or $8 to Colony as well. Flexibility is worth a good bit on its own - say I expand a Pot into a Bv & gain a duchy, being able to move either of those two cards next shuffle into a province is a pretty nice option.
Likewise, Graverobber cannot mill provinces which is one of the most clutch options for the Remodel family; yes you can gain a $5 back and then retrash it into a province next turn, but that requires either running double trashers (which is hard on actions and draw) or waiting a turn, a turn you often do not have. This can become extremely imbalanced with cards like Tr or Kc - Expand can target anything green for more VP/faster end game; Grave robber rarely is in position for solid Kc gains.
Colonies also tend to be a hard push for Expand, you can gain 4 VP with a play of Prov -> Col - more points with the same space efficiency.
Rebuild does seem to be more powerful than Expand, but that lies mostly in the digging/non-terminal part. In an actual engine (e.g. you got both from the Bm deck), Expand can be better because it works on cards already in hand (e.g. you have Quarry/Bm in play and buy a card, draw it and play it). If you have limited targets, Expand is much better if you draw it after your target (which happens around 1/n % of the time when you have n targets). Only because mass Rebuild is a thing does Rebuild make a whole lot of sense to buy instead of Expand.