Modern physical disks are intelligent devices, and giving them more parallel requests can allow them to optimize the head movement and lead to more global throughput at the expense of response time for an individual thread. Storage arrays and RAID levels can make a huge difference here, even if it is a single filesystem.

SSDs don't have any heads that need to be mechanically moved around, and they don't have to wait for the data on the disk to appear at the read head, so the seek times reduce to (nearly) zero. Therefore, you gain even more from parallel requests to SSDs.

Also, your application rarely talks directly to the disk. The operating system is also trying to optimize disk access, by caching and reordering requests.

Alexander