... What’s the point of holding a gun sideways?
To look Hollywood, of course. Journalists and gun experts point to the 1993 Hughes brothers film Menace II Society, which depicts the side grip in its opening scene, as the movie that popularized the style. Although the directors claim to have witnessed a side grip robbery in Detroit in 1987, there are few reports of street gangs using the technique until after the movie came out. ...
While the side grip does not increase the risk of stovepiping, it is terrible for aim. It’s extremely difficult to properly use the top-mounted sight on a handgun that is turned sideways. Not that this matters much to the average street criminal. According to an FBI study, 60 percent of them don’t even use the sight.
—Why Do Rappers Hold Their Guns Sideways?, Brian Palmer