What is the theme and the purpose of the writing of "the Lottery"?

An author's purpose in writing a story is generally
expressed in the theme.  In this case, Shirley Jackson wrote "The Lottery" in order to
express the theme of mindless adherence to tradition.  Let's face it.  The only reason
this town continues to conduct a lottery is because they've always done it.  Other towns
have done away with the practice--much to the dismay of the old-timers in town such
as Old Man Warner--so we know it can be done.  At one time, perhaps, the lottery was
somehow connected to a fertility ritual, sacrificing to the gods in hopes of finding
favor for the crops or the town or whatever.  Now, though, the practice is senseless and
even barbaric.  Why do they continue to do it, then?  Because they always have.  Oh,
it's changed some over the years--they use paper instead of wood chips, and many of the
rituals connected to the event have been lost; at its core, though, though, this is the
mindless, unthinking, unquestioning repetition of what has always been done.  The
question Jackson clearly asks is whether we have any mindless traditions which we adhere
to without rhyme or reason, and is that a good thing. 

When
I teach "The Lottery," I'm always reminded of the story about a mom who was preparing a
roast for dinner.  She would always cut the ends off the roast, and today her daughter
asked her why she did that.  The mother said she did it because that's the way her
grandmother always prepared her roasts.  Later, the young girl asked the grandmother why
she cut the ends off the roast before putting it in the oven, and her answer was
simple:  "Because it wouldn't fit in the pan."  All these years of cutting the ends off
the roasts for no good reason.  What a waste.  And that's the
point.

There's more, as this is a complex story, but this
is the primary theme the way I see it. I've also attached an excellent e-notes summary
of themes site below for some further insights on this story.