I'm not sure why you think staff officers carried them for combat. They were not really the ones responsible for the actual fighting in the first place.
More generally, Japanese officers carried swords as accessories for their uniforms. The blades were both ceremonial as well as status symbols, because higher ranks were entitled to swords with special distinctive styles. For instance, General officers used swords with: "brown and red strap with gold wire, zig-zag stitching and yellow tassel1".
It is tempting to attribute the carrying of swords to the traditional association of samurai and the katana, but this was not the initial motivation. The practice in fact introduced in the late 19th century as a part of the Meiji reforms. In her effort to modernise her armed forces, Japan adopted western style uniforms. The standard blade introduced in Meiji 19 (1886) thus emulated the western sabre:
The traditional image of samurai did have an effect, but only much later. Western styled sabres were replaced by more traditional designs decades much later in the 1930s, after the rise of Japanese nationalism. The standard issue of Showa 9 (1934) resembles katanas much more:
And the 1938 standard issue:
The katana style swords remained the standard into and till the end of the Second World War, with another version coming out in 1943.
1: Jowett, Philip S. The Japanese Army 1931-45 (2): 1942-45. Vol. 2. Osprey Publishing, 2002.