BOAC
Polar orbits are used by weather sats (NOAA etc) and those on surveys of some sort eg Landsat.
The advantage of a polar orbit is, given that the globe rotates within the plane of the orbit, that the entire surface of the earth may be photographed every 24hrs. The ISS, Shuttle etc orbit at 50Deg to the equator.
Now, it's a long time since I dabbled in this, so it's a case of E&OE but I think that orbital inclination is directly related to the latitude of the launch site and/or the size of the payload.
Sorry if the previous para is a bit vague but perhaps someone else can quote chapter and verse.
N o t a
Edited to add:
http://www.centennialofflight.gov/es...RBIT/DI154.htm