What's the use-case for gain selection on a DAC?
Simple use case: you want an output higher than the internal VREF (2.048V) but don't want to add to your parts count with an external reference.

The less obvious case is what the datasheet calls "multiplier mode". In this case you keep the digital signal fixed and feed an analogue signal into VREF. That gives you a programmable gain amplifier, and the 2*VREF mode lets you have a gain between 0 and 2. That could be useful for analogue control loops or sensor biasing.