I’ve been looking for a good answer to this question. Especially one that deals with all subject areas, not just English. And I think I’ve found one.
Mel Dixon used this quote in her Grammar workshop for the Hunter Region ETA:
‘Grammar has been stereotyped as tedious, unteachable and remedial… Grammar, however, is sometimes successfully taught, has symmetries and mysteries… and is an introspective and metacognitive way of thinking about our own ideas.’ (Michael Clay)
What I really like here (and what really makes sense) is that grammar is essentially a thinking strategy. Its value is not that it gives us ‘rules’ but rather ways of understanding. Works for me.