The purpose of this instrument is to show the direction towards the celestial North pole, from the location of the observatory.
Viewing along the top surface of the instrument, one should be able to locate the Pole star on a clear dark night.
The angle in the triangular wall is supposed to be equal to the latitude of the observatory location, which ensures that the inclined plane points toward the North Celestial Pole, where the Pole star or Dhruv tara is currently located, and hence the name of the instrument.