Why do we need wireless USB when we already have bluetooth? and vice versa? and beyond that, why do we need wireless HDMI?
Bluetooth is great, but it has it's limitations. It was designed to be a low power usage protocol for wireless devices such as mobile phones. It is great for small amounts of data transmission, such as with bluetooth headsets, GPS, mice, or keyboards. However, as soon as you try to send a few MB of data it starts to become unbearably slow. Most devices use bluetooth 1.2 which has a maximum transfer speed of 723.1 kbit/s, and the newest protocol can get up to 2.1 Mbit/s. So if wireless USB can transmit data at 480Mbps, and still have the same range as bluetooth, it would be roughly a 240x improvement for data transmission rate. Also, because it is "wireless USB" I'm guessing it would be compatible, or with little modification, existing USB drivers and be an easier transition (for product producers and consumers) to using more wireless devices. So specifically to comparing those two technologies, that's why.
Generally speaking though, IIRC various protocols arise as a result of patents and also marketing/business decisions. USB and Firewire are very similar, the biggest difference to my knowledge is that firewire was developed by apple and less widely adopted. You may also seen one of several articles on slashdot about HD DVD vs. Blu-Ray. Is there really all that much difference except who gets the patent licensing fees, and who is the one who lost billions in product development? Nope. It all comes down to business, not logic.