Shirley Jackson's short story "The Lottery" is a wonderful story for a variety of reasons. The foreshadowing is superb, and the twist at the end is fantastic. Additionally, the story is great because it forces readers to examine the practice of observing traditions.
Society, schools, families, teams, and other groups all have traditions, and those traditions dictate how and when certain practices are done. Unfortunately, the people that practice traditions tend to forget why a traditional practice is done, and in many cases, the set of circumstances that led to the tradition no longer applies. Jackson's story shows readers that a blind adherence to ritualistic tradition can be incredibly stupid.
There is a brief moment in "The Lottery" when Old Man Warner gives readers a hint as to why the lottery was initially put into place.
Used to be a saying about 'Lottery in June, corn be heavy soon.'
It seems that the people got themselves into a sacrificial cycle. Stone someone to death, and a good crop was to be had; but Old Man Warner's comment also seems to indicate that not even he knows why the lottery exists. That's an important point. In his mind, and in the minds of other community members, they do the lottery simply because the lottery is something that has always been done. They don't question whether or not they should still be doing it. They do the ritual because the ritual is what was done the year before and the year before that and every year for decades before that.
Unfortunately, this is what happens with many traditions. They are practiced because that is what the previous generations have done, and it is easier to continue the tradition than to buck the tradition and risk angering the previous generations. Rituals like the lottery continue to be held because it is easier to follow the status quo than break with tradition, and Jackson's story teaches readers that blindly following a tradition might not be a good thing.