We might use it

But Saul and the army spared Agag and the best of the sheep and cattle, the fat calves and lambs—everything that was good. These they were unwilling to destroy completely, but everything that was despised and weak they totally destroyed. 1 Sam 15:9

Would we do that?

Think about it. Have you ever pulled something back out of trash because you thought you might be able to use it some day? Or maybe you just can't seem to throw anything of value away. I grew up in a family that saved Christmas wrapping paper to be used again. "Be careful when you open that!"

But this wasn't going to be wasted. Why?

"However, anything specially set apart for the Lord—whether a person, an animal, or family property—must never be sold or bought back. Anything devoted in this way has been set apart as holy, and it belongs to the Lord.
Lev 27:28

It was of no value already, it was set apart, it belonged to Him. But it seems like a waste. What do you mean? How can you say that it was of no value?

I think we need to be constantly reminded of how BIG God is and that His ways are not our ways. But, we're not the only ones to ever ask these types of questions. Jonah asked that question about a plant going to waste (at least in his opinion) and here is God's response:

But God said to Jonah, "Do you have a right to be angry about the vine?"
      "I do," he said. "I am angry enough to die."

But the LORD said, "You have been concerned about this vine, though you did not tend it or make it grow. It sprang up overnight and died overnight. But Nineveh has more than a hundred and twenty thousand people who cannot tell their right hand from their left, and many cattle as well. Should I not be concerned about that great city?" Jonah 4:9-11

I think sometimes we forget how much our rebellion hurts God.

Here's the thing, Saul does not disobey God because he is so compassionate, so caring, so kind. He killed every Amalekite man, woman, and child, save one – the king. It was really self-serving.

Why?

He gets some popularity for allowing his men a good sacrificial meal with the Amalekite animals; it also means they do not have to sacrifice their own animals. Sparing Agag provides Saul with a trophy of his power, much like a stuffed and mounted animal.

Remember this is a lesson in obedience, so here's my question: are we keeping the best of material things and essentially giving God the leftovers?