With the standard system of coordinates, latitude and longitude, if you are not at the north or south pole you can give your position on the earth uniquely.
Notice that the longitude of the north and south poles are undefined.
(This is no mistake, it is not possible to find a nicely behaved system of coordinates for the sphere with one chart.
You need at least two.)
Anywhere except the poles you can give your direction of travel as an angle referenced to your line of longitude.
When you say, "I am traveling northwest," this is what you mean.
You have given the angle, it is $45^\circ$.
(This is often written as N $45^\circ$ W.)
If you know your latitude, longitude, and this angle, your journey is completely determined.
If you are at one of the poles, however, this method breaks down since your longitude is undefined.
You must reference some other axis.
It is natural to give your direction of travel by specifying which meridian you will take.
You might say, "I am walking south along the Prime Meridian, and so expect to go through Greenwich on the way to the south pole."