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What does that do, and is it necssary?
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An opto isolator with NPN outputs works by utilizing light to bias on/off transistors. The 4 FET control signals are connected to the anodes of the LED's at the input of the opto-isolator. The LED's then turn on their respective NPN transistor at the output of the opto-isolator. The NPN's are then used to amplify the signal and provide an adequate amount of gate voltage to fully turn the FET's on/off.
Is it necessary? Well, if you don't have a method of stepping up the voltage from IC1, you only have 3.4V to drive a mosfet transistor that wants 10V(with some power MOSFET's). So with an H-bridge that's driven by mosfets that never get fully turned on/off, you have serious losses in power which shorts the MOSFET's and your cells. I'd say it's pretty necessary, but there are other ways to do what I'm doing with the device that I've chosen. There are mosfet transistor driver IC's which do about the same thing but with a CMOS/TTL type device. ph2t was telling me about it once, but I forgot the part number. Ask him, I think he's using it on his newer nelly turbo.