The fat, and the other parts to be burnt with it. See Leviticus 3:3,4,9 7:31. So this was an additional injury; for lest the fork should mistake and injure them, they took such parts as they best liked whilst it was raw, when it might be better discerned.
Related Commentaries of 1 Samuel 2:15
1 Samuel 2:15
15 Also before they burnt the fat, the priest's servant came, and said to the man that sacrificed, Give flesh to roast for the priest; for he will not have sodden flesh of thee, but raw.