Did you set it to target specific country?
For SEO aspect, I think it's better to target country for those forum exclusive for your own country.
But, if you prefer more worldwide traffic, more advertisement revenue...
I guess left "uncheck" is good?
My site targets worldwide users, so I leave it unchecked.
Just to add, you'd probably want to use this if you're running your site under a ccTLD, like an .ly site that targets US visitors mainly. After some time Google should be able to detect the correct locale regardless though.