Why is the past perfect is used in this sentence?

Question

Uncle Vernon said sharply, “Don't touch anything he gives you, Dudley.”
The giant chuckled darkly.
“Yer great puddin’ of a son don’ need fattenin’ anymore, Dursley, don’ worry."
He passed the sausages to Harry, who was so hungry he had never tasted anything so wonderful, but he still couldn’t take his eyes off the giant. Finally, as nobody seemed about to explain anything, he said, “I’m sorry, but I still don’t really know who you are.”
(Harry Potter and the Sorcerer's Stone)

Answer 1

The answer to your question is no. If you removed but he still couldn't take his eyes off the giant, the rest of the sentence would remain unchanged.

Harry was so hungry that the food tasted better than anything he'd ever eaten before. The phrase he had never tasted anything so wonderful expresses this idea. In particular, had never tells us the sentence describes his entire life up until this point in the story. If the story were told in the present tense, it would say has never; since the story is told in the past tense, it says had never.

Answer 2

Tense in sentences is fixed with respect to a Reference time, which itself is fixed with respect to the Speech time, when the sentence is read. In this case the Reference time moves forward with the narrative, but it is fixed in the past relative to Speech time; in the passage at hand it embraces the time immediately before and after Hagrid passes Harry the sausages.

Consequently, the past perfect had ... tasted signifies that nothing Harry tasted before that Reference time (in which he received and ate the sausages) had been so wonderful; the past tense couldn't take signifies that during that Reference time (in which he eating the sausages) he kept his eyes fixed on Hagrid.