What is a single drive RAID 0 array?
In the case of this implementation (the LSI SAS2208 controller), JBOD does not use on-board cache, single disk RAID0 uses on-board write-back cache. The ceph benchmark explains it in the test setup. The performance increase comes from caching not striping. Most RAID controllers allow you to setup single disk RAID0 or RAID1 as a way to support JBOD, this controller is a little different in that it also supports JBOD (without any controller cache).
There are cases where RAID levels can use less disks than you normally think, and still provide increased performance or redundancy. For example Linux md RAID10 can be used with two or more disks, including odd numbers of disks, unlike traditional RAID10 which would require four or more disks and even numbers. Linux md RAID10 with two disks is faster than RAID1.
RAID-0 on single disk is used mostly on SSD disks, which acts as Cache disk (Intel Smart Response). When you activate "accelerate mode" for any HDD or RAID Volume via SSD Cache, IRST driver converts disk to RAID-0 Volume. If you are not using whole disk space, there will be automatically created second RAID-0 Volume, which can be used for other data. My example:
I am not sure, why it has to be in RAID mode, as I see no benefit from it. Maybe for some specific operations there is minor boost.