Why did Cain and Abel make an offering to God in Gen 4:3-4?

We are not told. It is possible that they were taught to do so by God; it is not likely that they would decide to begin this practice in precisely this way without some direction, at least not if the practice were God’s will from the Fall. Since (as we will shortly see) they still seem to live near the presence of God, they might well look upon God as a worshiped grandfather figure, so that these would serve as gifts; but the word for “offering” and not “gift” is used, yet there is no explicit indication that the offering here serve as substitutionary atonement.