The pelorus is a device which allows the operator to take bearings when it would be impractical to do so from the compass.
This can be useful as it is required to be able to take a bearing from as close as practicable to 360 degrees as possible. This may not be possible due to visual obstructions or impractical access. Often this is the case when installing a compass on the bridge itself for example. This can also be useful to take a bearing from a point where there is no compass such as a lookout in the forward position.