As my son worked on his English homework tonight (about nouns, verbs, adverbs and adjectives) he observed "this work is silly. When would I ever need to know how to identify whether something is a noun, a verb or an adjective? " He wasn't protesting as he proceeded to complete his work without prompting but his comment made me think a bit. I always enjoyed grammar as a child and thought it was interesting to learn about the structure of sentences. But.... what is the purpose of learning about different "parts of speech"? Obviously, grammar is important and children need to learn to use correct tenses and correct "agreement" of pronouns and verbs, etc. But is an exercise such as my son did today (i.e. "underscore the adjectives in this paragraph") in itself useful? Perhaps -- to attempt to answer my own question -- it's useful because if children learn to identify adjectives and adverbs, for example, they might become more conscious of their own use of descriptive words? (I am not questioning the purpose of homework, btw. I simply became curious because of my son's observation, when I realized I didn't really know how to answer his question.)
The underscoring activity is just your son's chance to show he can identify the parts of speech. Once the pupils can do that correctly the teacher should move on to more advanced tasks applying that knowledge which should seem more useful to him. It is vital to be able to construct good English sentences and also being able to talk about language makes it much easier to learn modern languages (though it is possible without grammar it is harder if you ate not doing it by living among native speakers).
Surely there are loads of things we learn at school that aren't immediately and obviously important.
Why learn to spell when you can use a dictionary/spell check? Why learn what life is like in India if you don't intend to visit? Why learn equations of lines (this is what DS has struggled with this week)? Capital cities?
I suppose it's just knowledge, to stop you sounding like an uneducated buffoon when you go out into the world, and might also actually serve to spark an interest in one area of learning or another that you can take further.