Their control arrangements and limited range also mean that mortars are most suited to direct supporting fire.
From the en.wikipedia.org
Typically these are general supporting fire units, but direct supporting fire units are also used if they are available and not fully occupied by their primary role.
From the en.wikipedia.org
The entire insurgent force began to direct its fire at Sgt. Miller as he continued to move forward, giving the Green Berets behind him a chance to take cover and give supporting fire.