What does chkdsk do, exactly?

One way to think of it is: /F fixes software problems, /R software AND hardware, since bad sectors are actual physical damage to the disk surface. /R does essentially what the old SpinRite program used to do (“used to” because today’s large capacity drives are for the most part beyond its abilities to deal with); analyzing every single sector on the drive, whether it contains data or is completely blank, and attempting to read AND relocate data it finds onto good sectors. Then it marks the bad sectors AS bad, so that Windows does NOT use them again, effectively “removing” the bad sectors from the system.

If you use CHKDSK /R that means that you are automatically using CHKDSK /R /F. The /R mean that everything that /F does is done as well as anything extra that /R does. Since everything that it does has already been accounted for, there is no reason to use the /F at all.