Often you want to use it in your upkeep to look for a card to Dredge, but if you already have the exact card you want to dredge in your graveyard, its better to dredge first so that, if you hit a Narcomoeba, you can Shriekhorn in response to the trigger to look for an Amalgam to return as well
You can save a high value target from removal with this spell, but more than that, it gives you way more than one mana of value particularly because of how good the Rebound effect is
Once on the board, their abilities allow them to generate massive amounts of momentum while protecting each other from opposing challenges
Theres nothing quite like ol Gary here, except maybe Kokusho, the Evening Star
So whether they are being challenged, challenging, being damaged by items, songs, or effects