Normally, you don't.
However there are some circumstances that might require it.
You want to upgrade to a new processor that has came out recently, and your current BIOS does not support it, but there is a new revision available for download that does. This is the probably the #1 reason people need a BIOS update. Another reason is perhaps your motherboard was a first revision, and there were some problems or glitches found when trying to use certain hardware with it, and the company corrected it with a later BIOS revision.
However, the old saying "if it ain't broke, don't fix it" is very good advice when talking about your BIOS. If you are having no problems, there is nothing, absolutely nothing an update (and it is an "update", not an "upgrade") will do for you. Many times, people update their BIOS for no reason other than there is a new one out, only to find out the new revision is causing them problems. There is also the possibility that when you do the update, something goes wrong. If you mess up here, it can leave you with a dead motherboard, and there is nothing you can do to fix it. You have to send it back the manufacture to have a new BIOS flashed, or if your BIOS chip is removable, you can call the company and have them send you a new one.