The motherboard has a number of chips that perform whole bunch of useful functions such as controlling drives/audio and video/network/usb etc, and each mb uses an unique combination (set) of such devices that need their own drivers and other files. That is why when you purchase a mb, you get among other things, a CD with these files.The manufacturer of the mb usually updates these files at periodic intervals - so when you reinstall, unless you have these files, some of the PC's functions may not work or cause problems - so either have the original CD handy or download the lastest drivers. While Windows XP has this problem resolved to a great extent and even searches the web for them, it still needs the latest/optimized drivers to work well.
It is difficult to get to the web and get these files when you are in the middle of an install.