"Now go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass." (1Sa 15:3)
"But the people took of the spoil, sheep and oxen, the chief of the things which should have been utterly destroyed, to sacrifice unto the LORD thy God in Gilgal." (1Sa 15:21)
"And Samuel said unto Saul, I will not return with thee: for thou hast rejected the word of the LORD, and the LORD hath rejected thee from being king over Israel." (1Sa 15:26)
God had instructed Saul to completely destroy the Amalekites, both the people and all of the animals. Instead of precisely following what God said to do, Saul decided to spare Agag and to keep the best of the animals as well. Saul sought to justify this action by using the animals as a sacrifice. Saul's claiming that he had done a good thing by offering sacrifices was no excuse for his disobedience. The end result was that the Kingship over Israel was taken from Saul and given to David. That is what happens to disobedient servants.
The meaning then is that we do not substitute something for God's command, no matter how "good" the substitute thing is.
There is a parallel to this in the parable of the talents (Matthew 25:13-30).
Then he which had received the one talent came and said, Lord, I knew thee that thou art an hard man, reaping where thou hast not sown, and gathering where thou hast not strawed: And I was afraid, and went and hid thy talent in the earth: lo, there thou hast that is thine. His lord answered and said unto him, Thou wicked and slothful servant, thou knewest that I reap where I sowed not, and gather where I have not strawed: Thou oughtest therefore to have put my money to the exchangers, and then at my coming I should have received mine own with usury. Take therefore the talent from him, and give it unto him which hath ten talents. (Mat 25:24-28)
Just as with Saul, the disobedient servant attempts to justify and rationalize his actions. In doing so, he puts himself above his Lord, usurping an authority that doesn't belong to him. The justification is in the form of, "surely he didn't mean that - I'll substitute my own understanding and do something good for my master."
That's the problem. The end result in both cases was that the servant lost his position and was discarded.
True obedience is an expression of faith. To be obedient means first having total trust in the other's commands. This is what God wants from us: to trust Him completely. Yet, man continues to substitute his own understanding. Man will substitute good works, moral behavior, physical safety, financial gain, or some other such thing for faith, all the while justifying such action by how good or necessary the action is. And, that is the same as substituting "sacrifice" for "obedience."
Obedience is the essence of the life of faith