It is my opinion that Abel kept flocks for all the practical reasons of milk, clothing and yes, meat. A diet of vegetation alone does not provide sufficient protein. But, of course the story is that Cain and Abel apparently became of 'accountable' age at the same time. Did Abel, apparently 'out of the blue' decide that the animals would be an acceptable sacrifice? I don't think so. If as in Hebrew law 'accountable age' is about age 13-14, then it seems to me that he must have had some help in his flock-keeping. I vote for Adam's instruction. Because in fact, Abel did NOT make the first recorded animal sacrifice.
Because, after all, Adam knew that animal sacrifice is needed and God requires a blood sacrifice. True, one may say, but the law was not yet given about the blood sacrifice! Yes it was, my friend. It was not only previously written, it was also demonstrated by Christ Himself, and used to forgive the sins of Adam and Eve. How can we think Abel knew?
"Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them."
It does not require a lot of thought to see that those animals couldn't give up their skins without losing both their lives and their blood. Thus, in plain sight it is written in Gen 3 that Christ Himself showed Adam and Eve the (then) acceptable requirement for forgiveness. In this case it was both a physical "covering" as well as a spiritual one acceptable to God. Now think for just a moment. Can you imagine what an impression this act must have made on Adam and Eve? They had NEVER seen ANY death up till this time, yet here is some totally innocent being shedding it's blood for THEIR sins. Oh yeah---they knew alright...and it is obvious that Abel listened to mom and dad..he tended the critters.
Does this first sacrifice remind us of something? It does remind me of Christ who came through Adam and Eve's offspring to pay for all of the sins of all mankind, He being declared "the lamb of God" at His Baptism.
The punch line? Surely, Adam and Eve told Cain and Abel what God expected, but Abel alone believed, and worked toward it. That is why Abel did it just right (Gen 4:4). Cain surely was told, but just did not accept. He wanted to do it his own way.