What important things can be learned by studying the history of the English language?

Well, nothing particularly useful, there is a book out there called the 'Story of English' which is interesting simply as a speaker of English. The transformation from Old English to Modern English is shocking. I personally find it amazing that a language over the course of 1,000 years can change so much to the point where a speaker of the later version would not understand the speaker of an Earlier version. Whether you consider that important is another matter, personally I don't think its very 'important' at all.

Answer

You shouldn't say things like "Well, nothing particularly useful", that's far too personal, you may confuse people. A lot can be learned from studying anything, and also the history of the English language. I do not see how you can say it is not useful. To study language in this way could help understanding in different fields such as anthropology, psychology, philosophy, history itself and of course etymology, which is the study of word origins and can be useful when linked back to anthropology etc. The logic which people often use to understand their environment, can be seen in the creation and expression of words. I think this is all logical but if someone can find suitable accreditation it would be great.