Old Man Warner reveals the closest answer possible to this question. He comments:
"Used to be a saying about 'Lottery in June, corn be heavy soon.' First thing you know, we'd all be eating stewed chickweed and acorns. There's always been a lottery,"
This use of a tired out phrase that is only remembered well enough to be shard by the seemingly oldest character in the group illustrates both the message of the entire story and the purpose of the lottery originally. It seems that the purpose had to do with the fact that people once believed if someone was sacrificed in this manner, the corn crop would come in strong. The message of this story has to do with doing something just because that's how it's always been done. The absurd message drives audiences to the point of horror making them consider their own ways.
While the answer to this question is not indicated in Shirley Jackson's "The Lottery," since the ritual is so old, it is suggestive of a drawing of lots to have one person be a scapegoat for the evils that may plague the future harvest. The concept of the scapegoat is an ancient elimination rite which involves the sacrifice of one as the vehicle of all the evils that may plague a community.
As support for this idea, there are two things that are mentioned. Clyde Dunbar is home with a broken leg and cannot draw a lot because in ancient rituals the sacrificial victim must be perfect; then, Old Man Warner mentions the connection to insuring the harvest when he recites, "'Lottery in June, corn be heavy soon.'"