SCL and SDA Pins of UNO

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Those pins were added in the "Rev3" version of the Uno. According to this, the reason is that, unlike the Uno, pins A4 and A5 on the Leonardo are not SCL and SDA so that meant shields with I2C devices would not be universally compatible with any Arduino board if they connected to pins A4 and A5. I believe the same issue applied to the Arduino Mega as well. By breaking SCL and SDA out to dedicated pins that were in the same location on any board, shields with I2C devices were able to be compatible with all "Rev3" style boards.