This permits the block layer to be removed from the kernel if it's not
needed (on some embedded devices for example). If this option is
disabled, then blockdev files will become unusable and some
filesystems (such as ext3) will become unavailable.

This option will also disable SCSI character devices and USB storage
since they make use of various block layer definitions and
facilities.

Say Y here unless you know you really don't want to mount disks and
suchlike.

source code:

block/