It is always difficult to know exactly what an author meant to do or what idea(s) they meant to convey with a particular text. unless we actually have a record of them saying so themselves. Beyond knowing authorial intent, we have to work to figure out what a text actually does accomplish.
This particular story seems to convey a few, related ideas. Namely, we human beings have a very hard time dealing with change. We tend to avoid it whenever we can. Next, we often hold on to outdated, sometimes cruel traditions or standards because they are familiar and they are what we've always done. Because they are what we've always believed, we are loathe to let them go.
Many characters seem to exemplify this theme of clinging to cruel traditions. Certainly, Old Man Warner seems to be just such a person. He gets downright angry when he hears talk of other towns doing away with the lottery, though it serves no real purpose, and he's even irritated by the casual demeanor of Mr. Summers. Mr. Summers has proposed making a new box to use for the lottery since the current one is cracked and splintered and the paint is chipped. In response to this progressive bent, the narrator says, "no one liked to upset even as much tradition as was represented by the black box." Not even the visual symbols of the lottery can be challenged.
Though there seems to be some vague notion connecting the lottery to the growth of crops, that certainly is a superstition to which most people no longer adhere. Thus, the brutality of the lottery is only enacted because it always has been, at least as far back as folks can remember. It is dangerous, we see, to hold on to traditions for their own sake. We must learn to analyze our behavior and change, or we will do irreparable harm to ourselves and others.