Take a look at the picture below. It is of a selection of loose change that I had lying around. It is not a random selection, as it contains one of each of the British coins under a pound. The total adds up to 88p. Four of the coins are silver coloured and two of them are copper coloured. Four are round and two are not. Is there anything else that can be said about them? Namely, why are they positioned the way that they are?
I apologise to my United Kingdom readers, as for them the answer to the above question is really quite obvious. However, it does also help to illustrate a valid and important point about writing, which I will come to in a moment. You see, it is very rare for a story to follow a single linear plot. It is possible, but in most cases it would be rather dull. To hold a reader's interest, a book must normally contain several subplots running alongside the main story arc. These subplots may not seem as important as the primary plot, but if they are to fully justify their inclusion, the story will not work without them.
Sometimes a writer will add subplots in order to bulk out a novella into a much more lucrative novel. This is not good. In fact, I would go so far as to say that these authors are cheating their readers. Look again at the picture of the coins above. What would happen if one of the coins were to be removed? The picture would have a very different shape, but would it have any more or less meaning? The answer is no, because the picture did not have any real coherent meaning to begin with. It is a collection of relatively disparate parts. If you can remove a subplot (or even a character or scene) from a novel and not effect the overall meaning, then it has no business being included in the first place.
Essentially, what we have in the picture above is not a novel, but an anthology of short stories (metaphorically speaking). Lennon and McCartney may have made a career out of patching together unfinished songs that each had written in order to create a whole (I've Got a Feeling from Let It Be is a brilliant example as is A Day in the Life from Sgt Pepper or We Can Work It Out), but to try this in a novel is just cheating. There is one particular famous author whose books I will never read again after catching them doing it so shamelessly.
In a novel, all of the subplots should be there to create a much richer overall reading experience. Every character, every scene, must effect the whole so that its meaning would not be the same without them. Now I want you to take a look at the picture below. At first glance it may appear no different to the one above. There is a slight difference though. I have flipped each of the coins over. Now they no longer seem so random and disparate. In fact, now I would say that they make rather a good metaphor for a novel.
For anybody who has still not spotted it, here is an image of the final coin in the set.