Well...2 things are sure
You weren't there
I wasn't there
So we can't know for sure
I might guess that it may help with recoil?
The sandbags are improvised armor for the crew…see FM55-30; convoys were usually ambushed and the gun trucks were on a defensive ROE.
The challenge in VN was the environment; the sandbags absorbed rain (weight) which negatively impacted vehicle performance.