One might ask the same thing of any Open source project.. What is the difference between MS Office and Open Office ? One is proprietary and the other is open so anyone can view the code and make alterations.
It becomes a bit more difficult when discussing Video drivers because ATI and nVidia have been keeping information about their hardware and drivers rather secret. the Opensource versions of the drivers do not support all the features of the proprietary drivers because of this. Certain Distributions such as Debian for instance, will not distribute and proprietary software with their version of Linux due to the
guidelines they have set down for their distribution. So when you install Debian, it will not automatically install the most functional proprietary driver for your hardware. Other distributions are more free with including proprietary drivers, codecs, and things licensed by other companies.
Intel on the other hand have been embracing Open Source a bit more with their hardware and have been providing functional
Open Source drivers for their graphics hardware.
The Linux community has been hoping for some time that computer hardware manufacturers will open up their design specifications and provide more support for those that write drivers, so more hardware will be fully supported in Linux without the need for
reverse engineering the existing drivers. Reverse engineering is the most common method for writing Linux drivers for most hardware at this point. A long and painful process compared to a cooperative design process with the hardware manufacturer.
Hope that gives a little more insight as to why Linux users request Open Source drivers..