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Precision and speed, reaction wheels can go from start, rotate you one degree and then stop all motion. With fans your going to end up constantly correcting because you never hit zero rotation.

The robot is exposed to the station airflow so it has to adjust constantly anyways.

While non zero, the spherical shape and minimal ISS airflow generally makes this a non issue. You can use those reaction wheels to calibrate very precise adjustments over time, you can also just bump into something to get rid of excess rotation.

Note, space telescopes effectively use both approaches.

You can get really good stability and precision using reaction wheels.

It may also minimize the impact all of that moving air has on everything else in the station. Without gravity, fans blowing your things around could get annoying.

Being able to operate with no atmosphere (e.g. outside of the ISS) would be an advantage I suppose.

Yet, the translation movements are done using fans. Perhaps it's a prototype and future version will operate outside.

Yup, I misread. I was thinking you meant what would be the benefit of not using fans at all.