Healing Salve allows you to save a creature from targeted damage such as Lightning Bolt or combat damage. But compared to those other four boons, it's a pretty mediocre card, since it's so situational. Still, while we look at it today as a pretty weak card, it wasn’t all that bad in the beginning. It’s also still such an iconic, classic card that it’s been reprinted numerous times, including in Duel Decks: Divine VS Demonic.
Despite being one of the more worthless cards in the original Alpha, Beta, and Unlimited sets today, Beta copies of Healing Salve sell for over $5 US and Alpha copies push $10. Despite being relatively weak even by early Magic standards, it was still playable back in the earlier days, and is still a collectible in black border.
When graded by PSA or BGS, Healing Salve is actually somewhat valuable. An Alpha copy in BGS 9 or higher condition can fetch over $30. On a side note, the rare “Summer Magic” variants of the Fourth Edition set includes Healing Salve, and a BGS 9 copy was receiving bids in excess of $70.