With this tool you can find the field of view, the exit pupil and the magnification for a telescope or binocular. This is for visual observing.
When doing visual observations it could be interesting to know the telescope's or the binocular's output pupil,
the eye's pupil diameter is about 6 mm in dark condition.
No value to have a bigger size exit pupil than that from the telescope or binocular.
You also get the f/stop and fov (field of view) seen through the ocular.
Compare with Moon's 0.5o (half degree).