Because it does not have built-in USB, it is a cost-effective and small Arduino® compatible board.

ATmega328 running at 8MHz with external resonator (0.5% tolerance)
Low-voltage board needs no interfacing circuitry to popular 3.3V devices and modules (GPS, accelerometers, sensors, etc)
0.8mm Thin PCB
USB connection off-board
Weighs less than 2 grams!
Supports auto-reset
3.3V regulator
Max 150mA output
Over-current protected
DC input 3.3V up to 12V
On-board Power and Status LEDs
Analog Pins: 8
Digital I/Os: 14

Sparkfun Details page

Required CP2102 or equivalent USB to be programmed