By turning the offer into a public demonstration, the Roaring Thunder Sect could spin the story as an alchemical consultancy, which many forces at its level would pay a fortune to obtain.
Also, there was a deeper goal. The Roaring Thunder Sect might not have alchemy masters, but many apprentices occupied its caves. They were self-taught, only having expensive books to rely on, and a public, prolonged, and interactive demonstration could change that.
The Roaring Thunder Sect wouldn't develop alchemy masters overnight, but improving its apprentices would benefit it in the long run, creating a permanent edge over the other Sects rather than just the temporary one given by Liam's products.
In a different situation or relationship, Grace wouldn't have hesitated to promise Beatrice the world, selling Liam in the best way she could, despite understanding that unspoken goal.
