The main thing that social stratification teaches sociologists is that they need to account for its existence. It teaches them that societies are always stratified to some degree and on some basis. Therefore, it shows that they need to understand why this is.
This leads to the development of various theories to explain the existence of social stratification. There is, for example, the conflict perspective which argues that stratification comes about because all societies have dominant groups that have won out in the conflict against other groups. There is also the functionalist perspective that claims that stratification exists because it is in some way necessary for society.
Social stratification, then, teaches sociologists that they must explain why it exists because no society seems to be without it.