This book by Nicolaides sat on my shelf for a couple of decades. I thumbed through it a few of times but never got much out of it. I didn't see the point - I didn't want to make drawings like the ones I saw in that book.
A year or two ago I picked it up again and this time actually did the exercises - and suddenly I was experiencing things I'd never felt before. I was approaching my own art in a different way. I started to see things differently.
We humans understand very little about how our minds really work. In the individual sense we understand very little about what's going on under the hood (or the bonnet, for my UK friends) at any given time. Fortunately as artists we don't really need to understand (maybe it's better if we don't) - we just need to find ways to get that mysterious, limitless source to give up its treasures. This is beyond what we normally think of as "creativity" - even the way we see and remember things is not what we consciously think.
I read in Art and Fear (great book, misleading title) that looking at a piece of art is not likely to tell you anything about the process that produced it (we're not talking about medium and technique). Having an artist walk you step by step through his/her process is not likely to get you to where s/he is. In fact, you will never get to where s/he is, because that's not your place.
That's why artists and art instructors have discovered and devised various methods, exercises, practices and mind games to help kick start the inner artist, to give you (what you think of as you) a taste of that other world, that other you, so that maybe once you've visited it a few times (or it's visited you) you have some chance of being able to reconnect with it reliably... at least kinda reliably.
As a student I didn't listen to my teachers very much. I have learned (and recent studies have confirmed) that in general we are really bad at predicting how something is going to affect us internally. We're really bad at predicting what we need. We pursue some things and avoid others based on faulty judgment (good for survival maybe, bad for happiness and growth). Often the thing that seems the most useless, or even harmful, is exactly what we need the most.
So some words of advice...
If you're a relative beginner - listen to your teachers and your artistic "elders." Give them the benefit of the doubt. Methods and techniques have changed tremendously but the journey of the artist hasn't changed much if at all. Don't take whatever they say as gospel - always test it out. There's a lot of misinformation out there, and ultimately only you can figure out how to make your art. But when they give you some seemingly useless exercise, or something that seems like it might be useful to someone else but not to you, take it seriously and honestly try it out. Invest in it, give it a chance, see if it pays dividends.
If you're more... mature, consider going back to some exercises or instruction that you may have ignored in the past, not seeing the value in them at the time. Maybe I could have appreciated The Natural Way to Draw when I was in my 20's, maybe if I'd been forced to check it out I would have gotten something from it, I don't now. In any case, revisiting these missed opportunities at a time when you may be on a long term plateau can be the source of tremendous growth for you as an artist. A lot of those exercises seem to scale to whatever level the person undertaking them is at.