This patch increases the number of visible garrison slots on medium walls from three to four and on long walls from five to eight.
Most short wall segments have a wall length of 12, medium 24, long 36. Infantry has a footprint radius of 1.5, i.e. a diameter of 3, which means short, medium, long wall can accomodate 4, 8, 12 units, respectively. Because footprints tend to be larger than the actors themselves, things still look good when footprints slightly overlap, e.g. placing them at intervals of 2, which means 6, 12, 18 units on walls. Nonetheless, let's be conservative and stick with intervals of 3. However, wall segments overlap with wall towers, which can be placed quite close to each other, thus the available space is only that part of the wall that's longer than the short segment, hence 0, 4, 8 instead of 4, 8, 12. (1, 5, 9 might also work.)
Roman siege walls are treated slightly differently, since their actors visibly consist of three, six, nine (short, medium, long) repeated elements; sticking with that looks better. Due to turret overlap, the outermost parts are unusable, which leaves four places for the medium and seven for the long siege wall.
The advantages are that it looks better and, if you have the units, makes defending walls somewhat easier, thus improving realism.
See also D2477, D2760, D2769.