If Williams genuinely are looking at other drivers and bringing others in for free practices or such, they could be looking for as wide a range of feedback as possible. Latifi has done the in-season test now, he'll have a feeling for the car. By doing practice sessions he'll also be able to provide feedback on new bits and improvements they make. They'll have Kubica, Russell, and Latifi to give their opinions. They'll get a bit less time each in the car, but 3 opinions may be better than 2.
Or, alternatively they don't believe in Kubica or Russell's feedback and are willing to cash a bit of money to put someone else in for feedback, and all they can get is Latifi. Plus he has knowledge of F2 cars, a much better car than that FW42 haha.