The end of John's statement is"....the lamb who takes away the sin of the world." Animal sacrifice
was the only way for people of that time to deal with sin. Since he was refering to taking away sins,
it seems practical that he would convey it in a way they could relate to.
Just one additional thought in reference to sacrifice that I think is interesting..... in Mosaic law
the people provided the sacrifice, but the very first sacrifice recorded was provided by God.
just as Jesus was provided by God for us. .....Genesis 3: 21 " The Lord God made garments
of skin for Adam and his wife and clothed them."
( Without the shedding of blood there can be no remission of sin.)