Why is there a wave spring washer and a plain washer between these two sealed bearings?
Ball bearings typically have some axial play or backlash

Take a look at the cross section for a ball bearing.



The inner and outer race are rarely manufactured to fully constrain the ball. If you take a bearing and hold the inner race, you'll notice you can move the outer race back and forth slightly. This motion would typically cause extra wear, friction, and vibration.

"Preloading" resolves these issues.

In the image above, the outer race would be pushed to the left while the inner race would be pushed to the right. This causes the ball to only contact each race at one point on the ball's surface. Connecting these two points creates an imaginary line, which is shown at some angle, known as the contact angle, relative to a radial line.

Both of the inner and outer races move together to reduce wear

Typically, you don't want slip between the bearing ID or OD and the mating surface, so the inner races of both bearings move together and the outer races of both bearings move together.

Two bearings help improve shaft alignment

A single bearing, because of the play mentioned earlier, can allow the shaft angle to vary. This is called shaft misalignment. Typically, bearings are spaced further apart, but using two bearings helps constrain the shaft misalignment greatly because the angular tolerance between two subsequent bearings is lower than that of a single bearing.

Putting this all together

The inner washer is used to space the bearing races apart so they don't rub (and wear the seals) while also coupling the motion of the inner races. This appears to be clamped rigidly with an acorn nut above the blades. This, however, doesn't constrain the outer races, which is why a wavy washer is used. Clamping the inner races together compresses the wavy washer, which forces the outer races apart, fully constraining the balls with opposing contact angles.