To answer the question that has turned up several times in my “searches” statistics:
Knowing your parts of speech allows you to write sentences that delight both your English teacher and your readers.
For example, if you don’t know your parts of speech, you won’t know when you’ve broken the “rule” about not using adjectives and adverbs. Knowing your parts of speech can make learning a second language much easier. If you get bored easily, then knowing your parts of speech can provide you with hours of entertainment–don’t just stand in line at the grocery store, copy-edit the headlines on the tabloids! Also, it makes you a kick-ass Mad Libs player.
In all seriousness, though, I have found that knowing an adverb from an adjective from a direct object has been the most helpful in learning other languages, as well as in my writing. For instance, the simplest sentence construction in Japanese is: [Noun] is [noun/adjective]. In Japanese, the verb always comes at the end of a sentence, and there is a particle that marks the end of the subject. This makes it dead easy to make up declarative sentences, such as: sora wa aoi desu. Sora ([the]sky) is the subject; wa is the subject-marker particle; aoi (blue) is an adjective; desu (is) is the verb. [The] sky is blue. (Japanese has no articles. On the other hand, German has at least nine.)
More complex sentences are just as simple (providing you have the vocabulary). Watashi no neko wa kuro desu. Watashi (I) is a pronoun; no is a possessive particle (essential an apostrophe-S); neko (cat) is a noun; wa is the subject marker — this makes ‘watashi no neko’ (my cat) the subject. What about my cat? Well, we already know where to look for the verb, and it’s the same as the first time, so we know that my cat is…something. In this case, it’s another adjective: kuro (black). My cat is black.
How about something a little more exciting? Here’s this one:
Watashi no neko wa neko no tabemono wo tabemashita. We already know what the subject is: my cat. Same construction as before, pronoun + possessive particle + noun being possessed. Then we have another possession: neko no tabemono. Tabemono is ‘food’ (literally ‘eating thing’, tabemasu = to eat, mono = thing), so it’s cat food. There’s a new particle, wo, which usually denotes that what came before is a direct object. And, at the end as usual, we have the verb tabemashita (ate; past-tense form of ‘tabemasu’).
So, in English, we have the sentence: My cat ate cat food. What was eaten? Cat food. (And yes, I suppose the narrowest answer is just ‘food’, but I’m not being that picky.)
Now that we know what goes in each slot, we can make up Japanese sentences all day long. Watashi no neko wa ninjin wo tabemasen. (My cat didn’t eat [a] carrot.) Anata no mimizu wa kowaii desu. (Your earthworm is scary.) Kore uma wa shiawase desu. (This horse is happy.) Anata no uma wa watashi no ninjin wo tabemashita. (Your horse ate my carrot. [Explains why the horse is happy.])
And that is just one of the reasons that it’s good–vital, even–to know your parts of speech.