Another way to understand, is this:
Say you have a weakened monster on your side cause of wall, which pretty much anyone has x2. With Potion you will gain LP equal to the nerf (-800 etc), but with Altar you heal the original attack points of your weakened monster.
Just out of curiosity, I had a game where I used this card on a set Hiro's Shadow Scout (yes, it was a dirty mill deck BUT I was going against a Toons player) and was wondering why I didn't gain the 600 or so LP from my set monster's attack?
Is there some yugioh rule that I don't know about that says the attack of a set card is zero and since Destruct Potion doesn't take the attack of the monster while in the graveyard I got nothing?