It's for ease of loading while in a coach or horseback or onboard ship. It doesn't really spread shot...which probably spreads pretty well anyway, though that was the myth at the time. Some are even oval shaped to attempt to spread the shot horizontally. And, of course, there is a SERIOUS intimidation factor involved.
The number one use for blunderbusses was naval warfare. Short, handy, powerful, relatively easy to load, and perfect for close quarters naval combat. The second use was what the above gun was styled for: coach defense (or offense...). England was not a paradise at the time, and highwaymen ruled the roads. They carried them too.
They are not exactly well designed for hunting, and not very commonly seen among the average shooter in Colonial America who might have a more appropriate fowler or rifle.