Advantages of the reference sector method

It is simple and fast.
Because the instruments used are in sun-synchronous orbits, the solar position is the same at a given latitude independently of longitude. Therefore, the light path through the stratosphere will be the same and the subtraction can be performed without having to worry about radiative transfer in the stratosphere.
Any real measurement is affected by biases and other systematic errors. Many of them lead to offsets, and as long as they are constant over the time between the measurement over e.g. Europe and the satellite overpass over the Pacific several hours later, they will cancel in the reference sector method.