The Scheme lets you take back an action (and allows you to play another action this turn, so you can use the other half of the combo), and the Herbalist lets you take back a treasure. These work well together because you can choose the pair.
The most obvious use is to take back the Herbalist with a good treasure, which increases its value. With Scheme/Herbalist/Gold, you will be rich for three turns. With Scheme/Herbalist/Potion, you can use the Potion three times, and at least the first two will come with a +Buy so that you can get a Vineyard/Transmute and something else.
A less common use is to get the right treasure/action pair with a trash-for-benefit action. One of the recommended Hinterlands/Alchemy sets includes Apprentice and IGG; if you draw the IGG after Apprenticing something else and have Scheme/Herbalist, you can Apprentice the IGG next turn to draw five cards. If you are taking back a Spice Merchant, you can take back the Copper that it wants to trash. At the end of the game, you can use a Gold this turn and then take back Remodel/Gold for next turn.