Collector's usually want their expensive rarities in the best possible condition. Obviously, some prefer sealed games, but others like to know that everything is in the box that's supposed to be there, and also that the game works fine, so some pay extra for a semi-sealed game that has been checked. Don't ask me why.
The beauty of semi-sealed games are that they are just like mint used games, but have the benefit of the box being protected by the plastic, thus keeping it mintier than mint, especially in the case of SNES or N64 games