Most of hardware drivers are built into linux kernel. It is always better when they are. But in some cases you can or have to install them separately:
1. Hardware vendor does not disclose protocol to interact with hardware and releases closed source proprietary drivers. In this case there is no way but to install them separately.
But frequently there are also alternative open source drivers that are created by reverse engineering. These drivers are integrated into the kernel but may perform not as good as proprietary.
Example: AMD and Nvidia video drivers, Epson printer and scanner drivers, etc.
2. Open source driver exists but is not integrated to mainline kernel yet. Or is integrated to unstable versions, or to unsupported by Ubuntu kernel versions.
In this case drivers can be obtained from PPAs or as source code from github or other places.
Example: Focaltech touchpad driver, Realtek bluetooth driver, etc.
3. Sometimes open source drivers in mainline kernel exist but do not perform as good as some alternative open source drivers developed by hardware vendor or community.
In this case it is an option to install alternative drivers.
Example: Realtek wireless drivers.
If some of your hardware is not supported, it is always good to report it to launchpad. Someone will take care of it and add support to linux kernel. Also co-operation with testing is helpful.
I would like to add an important note regarding installation of standalone drivers.
In most cases hardware drivers are built and installed as kernel modules.
If you build a kernel module from source, after kernel is upgraded the module will not work. It is built for a specific kernel version. You will have to re-install it after each kernel upgrade.
That is why it is always preferred to install drivers from PPAs with DKMS packages, or install them using DKMS manually.
In this case after each kernel upgrade the module will be automatically re-built and installed for a new kernel.
Bad example is installing Nvidia drivers from .run files downloaded from Nvidia site. In this case the driver is not registered correctly in DKMS and after first kernel upgrade you will have a black screen.
But you can always get all Nvidia drivers packed as DKMS from PPAs.