Oracle Block Size VS OS Block Size

I have a question why it has been recommended that the Oracle Block Size must be multiple of OS Block Size. Say in some Unix Installation the O.S. Block is 512 Bytes but Oracle Default Block size comes with 2K. Multiple of OS Block Size. When OS and Oracle Transfer the Data(Or Data Exchange takes Place) it must be in terms of OS Block.
Thanks to all of you.
Keshav

Yes, but say you have an OS block size of 4K. You certainly wouldn't want a db block size of 2K. You would be reading in 4K each time you requested a 2K block.