Those new to VEX may be a bit mystified about these black plastic pieces called bearing flats. To start with the basics, VEX shafts (axles) have a square cross-section with slightly rounded corners. VEX metal structure pieces also have square holes that are a little bit bigger than the shaft. So there's a square-ish shaft rotating around inside a larger square hole.
If shaft end is put through through a metal structure piece without a bearing flat, it will work, but it's really not good in the long term. They rattle, make bad noises, wear out your parts in a hurry, and don't do as good a job at transferring power from the motor to where it's needed.
The bearing flats have round holes that are just big enough for the shaft to spin around in, and keep the shaft centered in that square hole in the structure piece. Smooth quiet sound, no rattling, very nice. If a shaft is going all the way through a C-channel piece, two bearing flats are needed, one for each side of the C-channel. Every square metal hole through which a a shaft is fed needs its own bearing flat.
It's easy in the build process to forget the bearing flats in a rush to get something working. But later the builders will have to take things apart to put them in, and no one's happy about that. Using bearing flats is a habit that one needs to develop, like flossing your teeth. Bearing flats also nicely act as spacers, especially where you need spacing anyway.