Other than the obvious fun :)
and the fact that we have some folks who generously contributed the prizes
why have a proofreading contest?
Well- for example---- Genesis
we transcribed Genesis from a TINY printed Bible from the Edwin Archer's press.
We later found (online) a pdf a library had posted.
We wanted to compare the two with each other but had moved on to other projects- and felt we needed help-
your help- dear readers!-
and so the idea of a contest was born!
Almost immediately after the idea, the gifts were given-- so we had a prize
it just seemed fortuitous.
an added benefit: we know that people are using the Genesis to Study the language.