Which animals were allowed to be sacrificed altogether? Male, female, blemishes and not, etc. But what about a treyfa (an animal that was going to die within the year)? Plus, the Gemara probes the implications from this status to the animals that boarded Noah's ark. Distinguishing between "clean" and "unclean" animals may have been unclear before the giving of the Torah, but they could derive which animals were kosher and not by virtue of how many of each was saved on the ark (7 for the kosher animals, even if they didn't yet know that they would be "kosher" animals). Also, the Gemara eases into halakhot about non-Jews bringing offerings outside of the Temple, and on private altars that were not acceptable for the Jews (at that time). Plus, the possibility of lacunae in the biblical verses cited by the Gemara. And the Jews could advise the non-Jews as to how to make the offerings, but not do it themselves.