English is treated like a school subject but could as easily be called something like Communication.
English, or our mother tongue, or whatever language we use allows us to:
Narrate stories, things imagined
Inform other people, as a witness in court perhaps.
Describe things, an experiment in a lab perhaps, or something beautiful that you want to sell.
Instruct someone, how to assemble a piece of furniture, or how to avoid a threat or risk.
Persuade someone that they should do something that they were not planning to do.
Convince someone that they ought to do something.
Contract, making a commitment, undertaking an obligation.
When we do these things face to face, we can ask the other person to clarify things if we do not understand what is said. However, it is often important to write things down, to make a record. Then if there is confusion or uncertainty about what has been written we are rarely in a position to ask the writer for an explanation.
For this reason alone it is so important to present our ideas clearly and in ways that encourage a reader to continue. Not only will poor writing fail us, for our message fails, but it is a kind of bad manners and will fail our reader.
This what good teaching should aim encourage, and not just in English lessons.
I can show how a single unnecessary comma in a contract cost a company millions. Take a look at What, How and Why: a manual of better English. http://www.peterinson.net/better-english/