When shall a current output DAC be used?
I'm going to answer this for just those DACs that contain an R-2R network that converts a digital number to an analogue value. Here's a current output DAC of this type: -



Interestingly, the R-2R network can be used a number of ways to get the same result. here's a different method but this time it uses a current to voltage converter made from an op-amp: -



Here's another play on the same R-2R theme. This time a high impedance buffer amp is used: -



So, you can either have an un-buffered current output DAC and choose to convert that output to a voltage OR you can have a DAC with the voltage buffer built in. In extreme situations you may decide that the internal voltage buffer isn't as good as you want and you might choose an un-buffered R-2R DAC and do the conversion yourself.

There are also those occasions when you might want to build a digitally controller multiplier - maybe you want to alter the amplitude of an analogue signal - if this is the application then an un-buffered 2-2R DAC is going to be the one you choose. These types of DACs are generally called multiplying DAC: -



The above is a 2 quadrant one but you can make 4 quadrant types: -