Could anybody please describe me how a teacher's knowing about the history of English language can help him explaining things to his students in an EFL classroom?

I am studying about the history of English language. It is really interesting to know how this wonderful language changed with the time. While studying it I read that a teacher's knowledge of English language history can help him to explain things to his students in an EFL class. I have no idea how??? I guess if a non-native speaker listens to some old English which is not common in use and he never learnt it before...perhaps to avoid that confusion it is important to know about the history too.

Please describe it.

Thanks in advance.
I suppose the main help is a knowledge of the various languages that have made contact with and make up English-- Latin, Greek, French, Anglo-Saxon. Identifying the sources of roots and affixes and seeing how they recur throughout the vocabulary is a great aid to finding some logic in the madness-- and is helpful in remembering the words too.
Teachers: We supply a list of EFL job vacancies
It's also useful to know about historical influences on the language.

In the 18th and 19th centuries, for instance, many words and phrases of Indian origin were absorbed into English, and are still current - especially in corrupt forms.

To take one small example, the slang phrase 'big cheese' ('important person') derives from 'chiz', Hindi for 'thing'.

Then you have Shakespeare & the King James Version of the Bible, which have both contributed dozens of phrases to everyday English: 'Jesus wept', 'one fell swoop', etc.

But I want to know how practical and useful it is, for a non-native speaker to have knowledge of the history of the language he is learning.

Could anybody please explain it?