The sysfs filesystem is a virtual filesystem that the kernel uses to
export internal kernel objects, their attributes, and their
relationships to one another.

Users can use sysfs to ascertain useful information about the running
kernel, such as the devices the kernel has discovered on each bus and
which driver each is bound to. sysfs can also be used to tune devices
and other kernel subsystems.

Some system agents rely on the information in sysfs to operate.
/sbin/hotplug uses device and object attributes in sysfs to assist in
delegating policy decisions, like persistently naming devices.

sysfs is currently used by the block subsystem to mount the root
partition. If sysfs is disabled you must specify the boot device on
the kernel boot command line via its major and minor numbers. For
example, "root=03:01" for /dev/hda1.

Designers of embedded systems may wish to say N here to conserve space.

source code:

drivers/pci/slot.c