What's the "/sys" directory for?

/sys is old. It was introduced before the Linux kernel reached 2.6 (back when there was a 2.4/2.5 split). Since the first Ubuntu release used a 2.6 kernel, every version of Ubuntu has had a /sys.

/dev contains the actual device files. It does not provide access to all devices that the kernel knows of (such as ethernet devices, for one - Why are network interfaces not in /dev like other devices?, Why do Ethernet devices not show up in "/dev"?). It is an interface to the device itself - you write to the device, read from it, etc.

/sys is an interface to the kernel. Specifically, it provides a filesystem-like view of information and configuration settings that the kernel provides, much like /proc. Writing to these files may or may not write to the actual device, depending on the setting you're changing. It isn't only for managing devices, though that's a common use case.

More information can be found in the kernel documentation:

Top Level Directory Layout
~~~~~~~~~~~~~~~~~~~~~~~~~~

The sysfs directory arrangement exposes the relationship of kernel
data structures. 

The top level sysfs directory looks like:

block/
bus/
class/
dev/
devices/
firmware/
net/
fs/

devices/ contains a filesystem representation of the device tree. It maps
directly to the internal kernel device tree, which is a hierarchy of
struct device. 

bus/ contains flat directory layout of the various bus types in the
kernel. Each bus's directory contains two subdirectories:

    devices/
    drivers/

devices/ contains symlinks for each device discovered in the system
that point to the device's directory under root/.

drivers/ contains a directory for each device driver that is loaded
for devices on that particular bus (this assumes that drivers do not
span multiple bus types).

fs/ contains a directory for some filesystems.  Currently each
filesystem wanting to export attributes must create its own hierarchy
below fs/ (see ./fuse.txt for an example).

dev/ contains two directories char/ and block/. Inside these two
directories there are symlinks named <major>:<minor>.  These symlinks
point to the sysfs directory for the given device.  /sys/dev provides a
quick way to lookup the sysfs interface for a device from the result of
a stat(2) operation.

For example:

• One way of setting the brightness of a laptop monitor is:

  echo N > /sys/class/backlight/acpi_video0/brightness
  

• To get the a network card's MAC address:

  cat /sys/class/net/enp1s0/address
  

• To get the current CPU scaling governors:

  cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
  

And so on...