Pure speculation - a typical siege strategy was to dig under walls to cause them to collapse. First, the earthen mound would make the tunneling to collapse a much more labor intensive effort. Second, if an enemy was at the base of the wall it could actually be easier to hit them with projectiles at this angle rather than leaning over and aiming straight down.
Again, I have zero evidence to support these points, just spitballing here.
The tunneling issue makes sense. The wall will be much more stable because of the greater base area, and the sappers will need to dig a much bigger cavity under the wall for all the additional material to fall into - if the holes too small the wall might not collapse well enough for the ground forces to have a good opening to assault.
The second point is less convincing though. Forts and castle walls had mitigation for that extra issue - machicolations are an example. Often, arrows wouldn’t be used for killing the people right at the base of the wall, instead rocks or hot sand would be used to fuck up their day. These also took out armored units - rocks just, well, crushed them, and hot sand got in the gaps and visors and burned the shit out of them. They could also often not get rid of it without taking off the armor, so they just burned till the sand cooled down.
Also arrows were a manufactured commodity. Rocks were just taken from the land, or could be waste from quarries etc, and sand is rough, coarse, and everywhere.
