It is basically the /dev/VGname/LVname and /dev/mapper/ ,that it is being created when you create the and LVM structure. It called device node since it is under the /dev node.
So the question is, why is the /dev node it there ? Simple answer is that it is intended to be used as a convenient why for you to specify the device in the fstab file and the like.
The udev system is responsible for the creation of this nodes, and here is some points of why you want it to create them.
The primary role of the udev device manager is to provide a dynamic
way of setting up nodes in the /dev directory. The creation of these
nodes is directed by the application of udev rules in userspace. These
rules are processed on udev events sent from the kernel directly as a
result of adding, removing or changing particular devices. This
provides a convenient and central mechanism for hotplugging support.
Besides creating the actual nodes, the udev device manager is able to
create symbolic links which the user can name. This provides users the
freedom to choose their own customized naming and directory structure
in the/dev directory, if needed. Each udev event contains basic
information about the device being processed, such as its name, the
subsystem it belongs to, the device's type, its major and minor number
used, and the type of the event. Given that, and having the
possibility of accessing all the information found in the /sys
directory that is also accessible within udev rules, the users are
able to utilize simple filters based on this information and run the
rules conditionally based on this information. The udev device manager
also provides a centralized way of setting up the nodes' permissions.
A user can easily add a customized set of rules to define the
permissions for any device specified by any bit of information that is
available while processing the event. It is also possible to add
program hooks in udev rules directly.
The udev device manager can call
these programs to provide further processing that is needed to handle
the event. Also, the program can export environment variables as a
result of this processing. Any results given can be used further in
the rules as a supplementary source of information. Any software using
the udev library is able to receive and process udev events with all
the information that is available, so the processing is not bound to
the udev daemon only.
In Red Hat Enterprise Linux 6 and its clones, the Device Mapper provides
direct
support for udev integration. This synchronizes the Device Mapper with
all udev processing related to Device Mapper devices, including LVM
devices. The synchronization is needed since the rule application in
the udev daemon is a form of parallel processing with the program that
is the source of the device's changes (such as dmsetup and LVM).
Without this support, it was a common problem for a user to try to
remove a device that was still open and processed by udev rules as a
result of a previous change event; this was particularly common when
there was a very short time between changes for that device.
