Why does simple plug and play hardware sometimes include drivers?

I have about 7 devices (speakers, mouse, etc.) that are all USB. They all came with CD's containing drivers for it. My question is, why do they? All 7 of these devices have run just fine on every computer I've ever owned (XP, Vista, Ubuntu, 7) without me even bothering with installing the included drivers disk. All the functionality works. My first thought was that it was just downloading it off the internet but even land-locked computers (i.e. no internet) were able to successfully utilize the devices without me installing.

Can somebody explain to me the purpose then for these devices to include the drivers disk?

Every device you own uses a driver to communicate with the OS. In most modern operating systems, the default built-in drivers are enough to cover a myriad of devices, such as keyboards, mice and speakers.

But once in a while, a device comes along that the OS engineers haven't designed for. I.e., my Fitbit - a USB/wireless pedometer. In that case, you need to install a driver in order to introduce the device to the OS and allow them to interact.

Another common scenario is a known device with a novel feature (such as a mouse with extra buttons, a keyboard with special function keys etc.) in which case the built-in driver will only cover the known features, and deprive you of the use of the extra functionality - until you install the dedicated driver.

The CD's might also contain supporting software that aren't drivers per se, such as control panel applets, testing/setup/diagnostic utilites, and .PDF manuals.

In some cases a device, such as a mouse, might have basic functionality covered by the base class USB driver, but may have extended functionality available if the manufacturer driver is installed.

The new Windows 7 "Device Stage" feature can display an image of several classes of connected devices. Windows 7 offers to download such information from the Internet but it is possible software CDs have that information too.