There are a few different reasons as to why an artist might choose water-mixable oils over acrylic paint.
The texture of the paint itself, the buttery thickness of oils is sometimes hard to replicate with acrylic.
Drying time is also a factor, you can blend longer with water-mixable oils than you can with acrylics (without the use of a retarder or medium).
Water-mixable oils can be used with regular oil paints, although this will revert the water-mixability.