How does the --bsize option in fio work?

Since fio is a benchmarking tool that for each run, should simulate a real I/O workload, how does the --bsize option fit with that? My understanding is that the filesystem has a set block size for which an application issuing a read/write operation has to use? The app wants to read let's say 256KiB of data. If the filesystem uses a block size of 4KiB then that would be broken down into 64 blocks. If I were to use fio to simulate this, but set the bsize to 256KiB, would that have any effect on the read operation? The filesystem wouldn't write 1 block but still 64 blocks, correct?