Rim joist is good to insulate, it made a world of difference in my garage. If the basement space is unconditioned and little used/unfinished you would want to keep the heat in the living space above it. Or you could do the basement walls if it is heated. Or if unheated you could do the parts of the block wall that are above ground. Cinder blocks have very low R-value 1.9-2.5 which if they are above ground makes them another place for heat to get out.
https://www.energy.gov/energysaver/where-insulate-home
Been about a decade since I had to buy insulation but it looks like a common number of batts is 8 or 11. I kind of remembered it wasn't a lot as the batts are only 48", so 4-5 wall bays. Anyway, the owner likely had extra and had a place to put it. You might find the same R-value and brand in the attic perhaps. Or a buddy gave it to them, it's a pain to store once you cut the package open. I had a few extra batts bagged up without a good place to really keep them for a while.
What I am wondering is why the previous owner skipped that bay. Is it not 16" on center?