The config above ensures the right information for the chip is used in creating the optiboot image.
No, you'll still need to build the optiboot image yourself (or use the existing .hex file.)
The info in the boards.txt file is used for compiling the user sketches, uploading the sketches via the bootloader OR via a device programmer, and for the "burn bootloader" command.
The usb-serial programmer is presumably a non intelligent hardware interface that converts usb input to ISP serial on MISO MOSI pins. In that case why the need for boot loader?
Most programmers are somewhat intelligent. They do things like check the device type, and convert "high level commands" (like "what device type are you", or "program these 64 bytes of flash at the specified address") to the appropriate AVR ISP commands on the SPI signals. All sort of necessary since there's no obvious exact translation between usb (or any other typical desktop PC interface) and SPI. (at any rate, there wasn't, in the timeframe when Atmel was specifying ISP protocols.)
The bootloader substitutes for the intelligence in the programmer, permitting the AVR to modify its own flash over a dumb data connection.