The Type 89 Grenade Launcher (aka Knee Mortar) is available in this light mortar formation primarily to allow it to be used for smoke discharge*
*
The "mortar" unit class allows for indirect fire, and for longer range smoke rounds (and more of them).  The Grenade Launchers included in the infantry unit classes cannot use indirect fire, and the smoke for those units are hard-coded as hand held smoke grenades, not mortar smoke rounds*