Hi and welcome here at CDF,
The Firmware of a DVD+/-R(W) drive contains a lot of important and for a â€œgoodâ€ burn required information (Media ID, write strategy, media specific write speeds,...) of CD or DVD+/-R(W) DL medias. The companies are updating the firmware to add new types of media, to change something for already supported medias (if they notice some problems) or to add new features (such as DVD-R DL support or DVD+R DL speed upgrades). You need to update the firmware when you want your drive to get support for all this changes and new medias.
Hope this answer is enough.