The most common argument made about why these sandbags were added were mostly around the idea that these sandbags were a great protection against Panzerfaust or Raketenpanzerbüchse 54 "Panzerschrek".
However, here's something that many people missed when trying to reason this; these sandbags had little to no effect when it comes to stopping these rounds. In fact, both the Panzerfaust and Panzerschrek required about 1 meter of air gap to substantially reduce the penetrating capability of the warhead, which effectively not only rendered a sideskirt useless, but also made the idea of adding sandbags a moot point as well.
The shaped charge warhead isn't actually shooting a "molten copper jet" as many believed, but rather copper disc in the warhead that was formed by force into a high-velocity superplastic jet/penetrator due to the the Munroe/Neumann effect; focusing of blast energy by a hollow or void cut on a surface of an explosive.
All you did was provide a false sense of security while overburdening the vehicle with extra weight which significantly shortened the lifespan of the tank's suspension.
There's the Panzerfaust/Panzerschreck threat, and then there's another more equal threat that the American tanks would face in the ETO: the towed anti-tank gun.
While not produced as numerous as a handheld shaped charge projectile launcher, the presence of an anti-tank gun in the combat zone is still numerous enough to cause great threat. Remember that the European combat zone wasn't mostly an urban combat zone, but also between bocage and open field.
Anti-tank guns were more numerous and more "conniving" than a tank since they posed even greater threat because of their low profile and easily camouflaged feature which can be hard to spot in the air or on the ground at a distance. And these sandbags were also ineffective in stopping rounds from the guns.
However, the sandbags sometimes worked in dislodging shells, especially when the moisture inside the sandbag froze during winter, making it as hard as concrete. Several accounts were recorded where high-velocity anti-tank rounds were defeated by frozen sandbags. But in doing so the moisture, if frozen, added more weight to the tank which increased the chance for the tank to breakdown.