Assuming you are not a flat earther, then the shortest distance between two points on the sphere is a straight line on a sphere. That translates to a circle on the normal flat map projection, giving rise to the name The Great Circle Route.
Why should a broadcaster be interested in great circle route calculations. Well because that is where you will point your hex array if one is interested in long distance (OTH) communication.
You have your 4x4 bank of Yagis or Helix and you either want to deliver a payload or pickup a payload. Being so directional the array has to be pointed in the correct direction. Of cause there is the additional complication of the vertical angle but that depends upon the frequency, time of day, atmospheric conditions and how many butterflies are fluttering wings in Timbuctoo. Complicated without a feedback reference.
The values are entered in degree, minute, second + compass format or click the button for Decimal entry.