You misunderstood the original comment xl97.
The DAC selected is a 12-bit DAC, you perform 2 8-bit transfers to it to load in data, 4 of the bits are ignored.
That's all.
Other DACs use the 4 bits for various purposes.
" The power supply current is a low 350µA when operating from a 5V supply, making the LTC1257 ideal for battery-powered applications. The space-saving 8-pin SO package and operation with no external components provide the smallest 12-bit D/A system available."
This collection of properties made it a good choice for your application.