Baitfish that school, like shad, do it to appear larger as a group, and to disorient predators.
Having an additional eye spot on it's side helps the shad in this disorientation.
Shad don't feed in tight balls, but in larger, loosely connected schools, following the plankton.
But when they are threatened by a predator, they form tightly knit balls. The same holds true for silversides, sardines, anchovies, and other similar schooling fish that feed on phytoplankton.
Bass hunt by sight and by vibration detection. Obviously, a large school of bait, in a tight ball, offers a much larger vibration profile than a single bait fish. And the added spots in the bait ball provide additional visual disorientation.
It's no accident that bass hang on the edges of bait schools, picking off individuals that stray from the main school, or drive them to the surface to break up the school and separate individuals, which are easier to catch.
The eye spot on cranks imitates the eye spot on a shad, and makes the bait seem more realistic.