Goutweed is a popular herb for seasoning Troll cooking, especially when used by the Troll chef Burntmeat. The trolls thought that they had already harvested all the Goutweed and had it in their storage until they learned that it was possible to grow it again when Burntmeat was preparing Adventurer stew, so in My Arm's Big Adventure you help the trolls learn how to grow goutweed again.
Trolls enjoy using Goutweed as a cooking ingredient to make their stews taste more appetising, they commonly use it when preparing humans and goats. Thus it is appealing to humans and trolls alike because Burntmeat said "it makes goat taste as good as human", and that "Human tastes plenty good enough raw if you add enough goutweed." as was part of Burntmeat's promise to never cook another human after the Adventurer so he alters the deal by preparing uncooked humans with goutweed instead.
After My Arm's Big Adventure, My Arm grows a herb called Hardy goutweed, from the Hardy gout tubers he obtained during the quest. This is a variety of Goutweed suitable for the windswept cold climate of the Troll Stronghold, hence why ordinary Goutweed cannot be planted in the player's farming patch next to it. The player is not allowed to pick the Hardy Goutweed.