I suspect she has a Rescue & Recovery role; what used to be known as a 'standby vessel'. (The GA appears to show rows of seats which would be a requirement for R&R.) If so, then she could spend long periods loitering when the continued use of the main engines would be inefficient. That's when the Aquamaster (if that's the make) azimuthing thruster comes into its own allowing the vessel to keep station with minimum fuel consumption and maximum efficiency. She may have DP or may not.
Similarly if waiting-off for backload over an extended period, the Master might well elect to save fuel by going to the azimuth only or, if needing a bit more oomph (technical term) while towing, the azimuth may supplement the main screws.
Barry M