The number 1 answer for this question begins by stating:
"People don't generally use hierarchical structures 'in the real world' -- it seems to be something that has been forced upon them, a technical remnant of the past."
What?!? That's crazy talk. This is an old question but I wanted to answer it because it's spreading a bit of misinformation. Hierarchical systems are actually pretty awesome and efficient. I don't know what this guy is talking about.
He also says that:
"What needs to be understood is the way that people recognise and organise things. Our brains don't work in a hierarchical way (without generating a lot of heat). Instead, we recognise things by similarity -- similarity of appearance, smell, taste, touch, etc. We see an apple, and we know it's an apple straight away. We don't have to think about it -- in a sense, it's a one dimensional way of thinking."
This is crazy too. Our brains don't organize information in a hierarchical system but if it did, it would work much more efficiently. Our brains literally cannot work in a hierarchical way so it won't generate a lot of heat. It simply can't do it. And, our brains do not work by similarity either. I take that back. It does a little bit of classification. For example, they might put red memories next to red memories but this system of classification is actually a minor, secondary method used by the brain. The brain actually stores information by chronological order. That's it. Do you, guys think that's efficient at all? This is why when you forget something, no matter how much you think about the events that occurred after you lost something, you will NEVER remember where you placed it. But, simply walk to place at the time right before you lost it, you'll remember where you put the lost object in a second. Every memory is linked in one direction - forward. You can't remember things backward, unfortunately. Try it.
Storing things chronologically might be appropriate sometimes but many times, it's not very efficient. Yet, that's how Evolution set things up. By the way, tagging has its good points but it has its bad points too. But that's a topic for another time.
I just wanted to make sure people understood that hierarchical filing systems are not some remnant of the past. They are an excellent way of organizing information. You can divide the number of files into 2, 3, or 10 different folders (4 is preferred) based upon a sensible category. Doing it this way, you can quickly whittle down the number of files until you finally locate your file. For example, if you have 10,000 files, you only need about 6 folder levels to find the file you want.
So, what's the problem? The problem is that the amount of information has increased exponentially. I don't mean it has increased exponentially from 20 years ago to today. No. It has increased exponentially each year for the past 2 decades. This is the problem of today. TMI. Too Much Information. Maybe, we should rename it Too Many Files. Whatever. This problem actually started at the advent of the internet. We had something called portals like Yahoo, AOL, and MSN. These were websites that helped people find the websites that they needed. Need to go shopping? Yahoo gave you a comprehensive list of where to click.
It was essentially a list of websites divided via a hierarchical system. But, the problem even then was the number of websites was literally growing exponentially. The portal up keepers could not keep up with reviewing which websites were the best and posting it on their website. Search engines were apparently needed and needed quickly. Now, here comes the ingenious part. It wasn't intentionally ingenious but just lucky but still an ingenious arrangement if it was done intentionally. Search engines found a source of labor that could quickly find the website that you were looking for. Nope, it wasn't immigrants or out-sourcing the labor to India. The source of labor was the Department of Y.O.U. You and everyone else found the website for you and everyone else! You and everyone else did the work for the search engines! The websites you and everyone visit most often is tracked by the search engines and pushes them up higher in the site rankings. Basically, Google is taking your work and you pay them for it! Apparently, it's billions of dollars a year. Ironic isn't it? Or, maybe the correct word is sick? I don't know.
If you read an introductory book on information science, they would immediately explain that hierarchical file systems are a great way to organize data. However, the main reason that hierarchial file systems are becoming less popular is that they start to become inefficient as the number of files grows large.
I'll go through a simple explanation. I forget the exact number but after a certain number of levels, it gets mentally strenuous and time-consuming to retrieve just one file. For example, say you have 50,000 files which is slightly large for a personal computer. You'll need at most 8 levels of folders to reach the deepest folder if each folder node branches into 4 other folder nodes. (4x4x4x4x4x4x4x4=65536) You'll probably need less folder levels but I am giving you the worst-case scenario. Just going through 4 levels is time-consuming but imagine having to go through 8 folder levels to reach the file that you want!
This is why tagging has been becoming a more important system of file organization. The number of files have increased dramatically. In servers, it's not surprising to find tens of millions of files. Are you going to search through them via a folder system? That would be nuts. Tagging is a way to get to the information directly. The only problem is that tagging comes with its own problems like creating unique names.