The main reason is simple. If you use an old PC laying around - you would require up to 100W Power in average - makes 2.4KWh per day power to use.
An Epia SP8000E System, with 2x2.5" HD Drives uses ~28W Power in full utilization mode - makes approx 672Wh in a day - ~3 Times less money to pay ... Note than in IDLE mode - I have a constant 18W = 432Wh power consumption.

However - other features come in I based my choice on: USB 2.0, Firewire, Padlock-capability (e.g. AES encryption engine), Passive cooling, S-ATA connectors and a well supported cpufrequency-scaling. Check out the Epia Power Simulator for computing the powerconsumption of a system you might want to set up.