They look at the inner workings of language. Why language works like it does, in other words. Think of learning about linguistics in the same way you’d learn how an internal combustion engine works. Linguistics is what’s under the hood… you don’t need to know it to drive a car, but it helps understand what’s going on if you ever break down!
So… TEFL teachers should find these useful in providing more of a background to their teaching and students, too, will find them helpful in understanding language at a slightly deeper level.