This business of word order in English is crucial because the language is so lightly inflected. So lightly what? In English, words or their endings don't change much to indicate grammatical function. There are some common examples: I becomes me in the accusative. She becomes her, and so on. But compared to many other languages, there is relatively little of this in English. Instead, it relies more on precise word order to convey accurate meaning, which is why I make such a fuss about it.
Here are two words that depend on one another, one being like a tennis serve and the other a return of serve. That means that they look alike but they are quite different. It's a difference worth preserving and insisting upon because it helps to make the meaning of what you are writing clearer. And that's what writing is all about: the nearest you can get to absolute clarity of meaning and accurate communication.
Now this is worth going to war over, in my view. I know, I know: it's getting close to a lost cause, but still. The reason that confusing "uninterested" and "disinterested" drives me mad is not just that they mean almost opposite things. It's also that there is no perfect synonym for "disinterested", so if we mis-use it we lose it. If there were five or six synonyms for it, I wouldn't care as much. But we are in danger of losing a really elegant word altogether.
The distinction between less and fewer is hardly worth going to war over but it keeps coming up again and again. So, while I think that there are more important things to worry about, the difference between these two is worth knowing. And using fewer correctly does make for better writing.
One of the distinctions that I get asked about a lot. You could explain the difference between these two words by explaining the grammar, talking about defining and non-defining relative pronouns (or, if you prefer, restrictive and non-restrictive). But I find that that scares off more people than it enlightens. My explanation isn't foolproof but it's easy to remember and will keep you right most of the time.