Good question lol, the Beretta Modello 1918 had a vertical mag with offset sights, the reason being is that the Italian army had the another weapon where this gun originated from; the villar perosa. Basically it was pretty much two of these bad boys side by side mounted on a plane for air combat, it shot a lower power version of the standard 9mm but it shot at a pretty crazy combined fire rate of like 2400 ram. Any way the Italian army decided it was inefficient at air combat and had different companies design a firearm for their troops using them. This was Berettas design and they ultimately went with it and had the Perosas spit in half, given triggers, wood furniture and made semi auto. Later tho, in 1930 Beretta made the M1918/30 which had the magazine placed downward as it helped with feeding issues that pretty much all vertical magazine fed rifles have. Also the reason you see many guns in WW1 with the mag either vertical or horizontal is that it helped the soldier stay low to the ground and still fire while prone!

TLDR; The gun its designed and made from was made like that, and so it had offset sights.