At this time, the sacrifices were still needed to cover sin. This is why Abel brought of the firstlings of his flock, and of the fat thereof. The firstfruits of the flock isn't just the first numerically. The first, in Hebrew culture, always had the most important and prestigious spot.
This spot of greatest imoprtance and prestige is why Jesus Christ is called the "firstborn of every creature," (Col. 1;15) This verse says nothing about birth. Jesus Christ has always existed, and always will exist, becuase He is eternally God. There is just no better term in English to refer to that position in the family of utmost importance and prestige.
Similarly, Christ is referred to as the firstfruits of those that sleep - in other words, die - in 1 Cor. 15:20. This doesn't just mean He was first to rise, although He did open the gates of Heaven for those who had been in Abraham's bosom. he is the most important, the essential one, as the next verse says. Just as by one man - Adam - death entered the world, so by one man death was conquered forever!
What Abel brought, then, was called for as a picture of what would occur later, just as every other proper sacrifice would be. We see Abel doing what God had shown Adam, and what he had shown his children. This, then, is why God "had respect" toward Abel and his sacrifice; it was a "more excellent" one. (Heb. 11:4) It typified just what God would do for mankind.
It also showed that Abel was righteous, according to that same verse in Hebrews. Of course, there is none righteous; no, not one. (Rom. 3:10) But, Abel had been justified by faith - faith that God would one day bring that promised rRedeemer to redeem everyone. Salvation has always been by faith, becuase it was impossible to keep the whole law; if anyone keeps it all, yet offends in one piont, he is guilty, and therefore condemned. (James 2:10) Abel's righteousness came by his faith in what was then the future death, burial and resurection of our Lord for our sins. He just didn't know how it would happen.
Finally, Hebrews 11:4 mentions Abel's gifts. This refers to his gift of the firstfruits. God's Holy Word tells so many awesome things, that we can only scratch the surface of them.
