A 1099-R is for the money Out. For instance, the issuer might not know anything about the rollover.

The 5498 is for the money In. It's how you (and the IRS) confirm the money was rolled over properly. You would use it for your due diligence that what your taxpayer is telling you really is what happened. An example would be if the Gross out is not the amount rolled (1099-R doesn't = 5498). An indirect rollover would result in some withholding that has to be made up in the (re)deposit, for example.

The FMV would be used for RMD and for conversion that is pro rata if there was basis from pre-tax to post-tax, such as Trad IRA to Roth IRA.