At the equator, a degree or minute or second of latitude is exactly the same as a degree, minute, or second of longitude. As we approach the poles, however, the longitude lines get closer and closer, until they all converge at the poles. Consider any continuous path along the earth's surface, or a craft following it. The angle between that grid and that line is the bearing of the path at that point. Which way along the path, and which lines of the grid are used, latitude or longitude, may vary from case to case.
L is than a degree of longitude at the equator.
Well, once we agree that all degrees of longitude are equal at a given latitude L, we can sum them all up simply by measuring the latitude circle at latitude L, and compare that to the length of the equator. The length of a latitude circle is proportional to its radius. The radius of a latitude circle at latitude L is R cos L, where R is the radius of the Earth. As a result, it sports a cosine-curve shape. Pure east-west distance between two points at the same latitude is completely correct.