Bonnie++ benchmarks three things: data read and write speed, number of seeks that can be performed per second, and number of file metadata operations that can be performed per second. Metadata operations include file creation and deletion as well as getting metadata such as the file size or owner (the result of a fstat(2) call).
The Bonnie++ benchmark is a great yardstick to see if you are getting the performance from your hardware that you think you should. You can search around for Bonnie++ results that other people have produced and published using similar hardware. If you are making changes to how your RAID or filesystem is created, Bonnie++ is invaluable for testing whether the changes you think should improve performance actually have a noticeable and positive effect.
Bonnie++ is a benchmark suite that is aimed at performing a number of simple tests of hard drive and file system performance. Then you can decide which test is important and decide how to compare different systems after running it. I have no plans to ever have it produce a single number, because I don’t think that a single number can be useful when comparing such things.