I'm going to say the word "lot" again because five times so far isn't enough. Oops. Six.
The advice is meant to be helpful, and it usually is. But sometimes. . . it sucks. Especially if you take all the advice and take it all literally. Here are some of the most common sucktastic tips I see:
1. Cut extraneous words from your prose.
Yes. I've read really flowery prose. And some of it is really bad. But you can't make a blanket statement about cutting "extraneous" words, especially from prose. Most prose is, by definition, extraneous. It is made up of the words that make us feel and internalize a story. It's not the plot, it's not character, it's not setting. It's the pretty fluff that separates literature from field reports. Let's look at a real example:
"The power of a glance has been so much abused in love stories, that it has come to be disbelieved in. Few people dare now to say that two beings have fallen in love because they have looked at each other. Yet it is in this way that love begins, and in this way only."
But, you know. You should cut extraneous words. So it should look something like this instead:
"You might not believe it, but people must look at each other to fall in love."
BORING. And ugly. And sarcastic. There's probably a book out there where this line would be wonderful, but Les Miserables isn't it. Don't get me wrong, I get it. We all need to edit, and we all need to be edited. But to tell someone that they should cut all extraneous words is really stupid advice. The better advice would be to tell people to make their words beautiful, powerful, and purposeful.
2. Cut everything that isn't part of your main plot.
Let's use Miss Watson's claim to fame as the example. If Harry Potter had cut everything that didn't directly relate to the plot, we'd have a middle grade trilogy instead of a septilogy (not a real word, stay with me). No love stories. No Weasley twins. No SNAPE. Half Blood Prince could be boiled down to about three paragraphs from Dumbledore, since so much of that doesn't really affect the actual, central plot.
The better advice here would be to cut anything that isn't interesting or able to be resolved satisfactorily. You have a random scene that is fun but doesn't do anything for the story? Cut. You have a scene that feels random but clues us in to something that will become important later? Edit it so it feels smoother, like part of the whole.
3. Don't use any words "filler" words or verbal pauses. They slow down the narrative.
Let's look at another direct quote example, yes? Yes.
“Anyway, I keep picturing all these little kids playing some game in this big field of rye and all. Thousands of little kids, and nobody's around - nobody big, I mean - except me. And I'm standing on the edge of some crazy cliff. What I have to do, I have to catch everybody if they start to go over the cliff - I mean if they're running and they don't look where they're going I have to come out from somewhere and catch them. That's all I do all day. I'd just be the catcher in the rye and all. I know it's crazy, but that's the only thing I'd really like to be.”
And if we cut all the verbal pauses and words that slow things down:
"I picture kids playing a game in a field of rye. I'm standing on the edge of a cliff and I must catch someone if they go over a cliff. That's all I do. I'd be the catcher in the rye. That's what I want to be."
Again, BORING. And voiceless. We lose all sense of his personality. Yes, it's more brief and clear and straightforward. But so far as letting me get to know the character and fall completely into his head, it's useless.
The better advice would be to, again, make your words count. Don't put them there because that's how you'd say something in real life. Put them there because they bring your character to life. There's a difference.
All three of these tips have to do with one thing: SPEED. The people giving these tips probably read/write contemporary mainstream fiction. Their books clock in under 75K and are plot-driven. And if that's the case, this advice is really good. If you're very concerned about clipping along at a specific pace, then word economy counts.
But if your pacing works, your word count is well within acceptable limits for your audience and genre, then you probably don't need to take any of this advice too seriously.