none of us would ever do this but -- imagine you're peeing, and you want to aim back behind rather than straight down.

ps i'm wary of 'should dos' when it comes to how you sit a bike. I like a really aggressive anterior tilt (as above) but that's how my glutes work best. Some excellent fitters disagree, and recommend you be square on your sit bones. I'd say comfort over distance comes first, and once that's locked, how powerful you can get follows.