The USB hookup is naturally quick - the Cortex processor drives the link at 3MBit/s. And it has crucial advantages for a development system. You can control the reset mode of the Cortex, rewrite its Flash memory, and drive its serial debug port.
The on-board MCU is an STM32 32-bit ARM Cortex with 32KByte to 512KByte Program Flash, 6KByte to 64KByte RAM, up to five USARTs, three SPI, two I2C, six 16-bit timer/counters, three multi-input 12bit ADCs, one 2-channel 12bit DAC, a multi-channel DMA controller, two watchdogs, 5V tolerant I/O, and up to 72MHz operation
The Arduino format adds multiple expansion headers with 3.3V and 5V available, extends the built-in USB debug interface with an ARM 2x10 pin JTAG connector, and adds flexible reset circuitry and programmable LEDs. The Cortino will operate in both stand-alone mode and as a USB peripheral.