I don’t know if you have found that generally the combination screws have the flat and spring washer , most of them are pan head and cylindrical(cap) head, countersunk head screws are basically without washers. I believe there are customers who want to understand this problem, so screw manufacturers will talk to you about the role of the flat and spring washer:

1. The role of flat washer:
① Increase the contact area between the screw and the machine,
② Eliminate the damage to the surface of the machine when the spring washer is unloading the screws. When it is used, it is necessary to use spring spacer firstly, then will use the flat washer. The flat washer is next to the surface of the machine, and the spring spacer is between the flat washer and the nut. Flat washer will be increased the force area of the screw, the function of the spring washer is to prevent the screw from loosening, it can act as a buffer when the force is applied.

2. The function of spring spacer
① When the spring washer is used, after tightening the nut, the spring washer gives the nut an elastic force to tighten the nut, so that the nut is not easy to fall off.

(2) Generally if you had used the spring spacer, will not use flat washer, unless it is to protect the fastener and the surface of the mounting surface will be used flat washer and spring spacer together.
(3) Flat washer is generally used to connect soft accessories and hard & brittle accessories. Its main function is to increase the contact area, disperse pressure, then will can protect the soft accessories.
You had known what flat washer and spring spacer do, I think you also know Why don't countersunk head screws need to be used with flat and spring washer together. Countersunk head shape are tapered, but flat washer and spring spacer are uneven, so flat washer and spring spacer are not used. Generally, the countersunk head screw will be screwed into the 90° conical countersunk hole, and the flat washer and spring spacer can’t be used.