Neutrals are the invisible foundation upon which your outfit is built. A really great neutral isn't "seen" at all, and can thus be reworn two or three times in a week without registering its presence.
You pick your neutrals from what looks best on you - black or white or brown or navy or grey - or some combination of those colors. The reason that the color seasons all A) give you a few neutrals and B) tell you exactly which colors those are is that you want the neutrals to match as well as is possible. When you're first building a wardrobe, you want to pick a couple of neutrals and go. Sure, someone with a lot of closet space, a lot of money, or decades spent working on the project can have a variety of neutrals to go-to, but that person doesn't need to read this blog.
I started out by saying that neutrals are invisible. This is true because that's what you use them for - to build your outfit - and it is not-true, because they shape the mood of what you're wearing. Let's take black pants for example. My mom, an executive for twenty years, has any number of pair of black wool slacks. Unless I'm comparing them, I really can't tell the difference at a glance. Why? Because minor style differences aside, they're all about the same quality - and she wears the same basic style all the time (it's the one that's the most flattering to her figure).
If, instead of her wool slacks, she were to throw on some black cotton leggings - would you notice? Of course. And those leggings - no matter how new - would not be appropriate for an executive to wear to the office. Likewise, if she traded in the wool slacks for some black jeans, she's going to look sporty, regardless of what else she wears. Quality also matters - she gets good quality black slacks that she *can* wear a couple of times/week without pilling, sagging, or bagging. They aren't couture (and yes, you'd notice a really sharp pair just as you would low-quality gear) but they're right for her. When she has that big meeting, you can bet she's breaking out the best she has.
Were she to radically change up the style of the black pants - say, to wear a very obvious wide-leg trouser, or maybe some bell-bottoms... the added style makes the pant instantly less invisible. This is good when you're choosing your second or third pair of black slacks... not so good when they're your only pair.
The "classic" shape is determined by what has been worn for the last decade more than it is determined by your body shape. This is a pity, but there you are. If I were to go to work in an office, I wouldn't wear my long A-line skirts, because the voluminous profile is not at all invisible in this fashion moment. There are subtle variations in the classic line of office wear skirts and pants, but they change slowly enough that you have to be a decade out of style to be obvious.
Neutral basics are where every style book in the world will tell you to put your pennies. Because while neutrals are invisible, they speak straight to the subconscious. A sharp pair of classic slacks in a great fabric with good details vs. something that doesn't quite fit and is a little shiny in places where it oughtn't... instant status changer.
So, you have your neutral blouse, your neutral shoes, your neutral pants and your neutral jacket. If you've been careful they all match and you can make a monochromatic look or you can start adding in your secondary neutrals (classic colors in classic silhouettes that aren't neutrals) for basic looks that can be accessorized to suit.
I've been using office wear as an example, because office dwellers have the biggest need to have a certain "look" all the time. Next up, neutrals for those outside the corporate jungle.
Last night I went tonal monochromatic - I wore a green skirt with a green sweater and a green tank. (Yes, upgrading "tank" to "blouse" is worthwhile ... but I haven't done it yet - put it on that list of mine, k?) With that outfit (secondary colors) I could have thrown on a strand of pearls and put my hair in a bun for a classic look - but I wanted to have a bit of fun. So I wore a faux rose in my hair, put red earrings in, and wore a rose bracelet. The greeter at church asked me if I was a flower garden! (Since that was the point of the outfit, I was pleased). Trading the accessories around, or trading in one part of that outfit for another would have made a totally different effect, a totally different mood - and *that* is the point of using neutrals to best effect.
I seem to have managed a wardrobe of ivory and navy neutrals with green as my secondary color. That surprises me a bit, I'm more of a blue person in my head... or turquoise. But it's so hard to find blues that match really well, and just forget turquoise unless it's from the same dye lot. (Worse than navy for that. You've heard - navy matches anything... except navy. Same goes for turquoise).