There are many parallels that can be made.
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Why would I need fur coat when a wool one is good enough? Oh wait, why do I need a wool coat when an animal skins or from vegetation is good enough? Oh wait, do I need a coat at all if I stay indoors always?
The answer is you may not need one at all. That doesn’t mean this particular approach is the only vision and a good solution human kind can apply at a large scale. In fact, human kind express its self so divers that clothing style differs even from town street to town street.
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Back to tech. A web page is a document. Well, a document means a text editor. Is it?
It could be. Again, less is more. But… less with style is even more.
And we see this everyday. Everyone has a point where it gets his own copy of a particular Office suite. And everyone does the same, always. It starts playing with every font, with every drop down color control, with every shape and produces odd things, odd enough to look at them and say: “man, it’s so ugly only its mother could love it”, right?
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The same with web sites. Now, if that everyone where to use just text, we all would be unaware of their professional level. Asking for more than text from one helps asses in less words and less lies their true tech level.
One thing you can see real proof here, on SPF. There are some you’d see strongly and courageously agreeing and disagreeing on all topics, granted that someone else has posted an working solution first, they could ride the coattails on. But the moment they start producing by them self, you can measure their “level” to the micron just by looking at those odd things.
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So, why don’t we use just text on web? See above. We don’t like to be confused with somebody else: type, style and message of the content we send across the Internet. CSS and Javascript make content and diversity go hand in hand. They allow you to “dress up”, put “make up” and add personality and professionality to your content. Often, you can tell a few things by a first look at one person. The same with a web page. It speaks volumes before one even starts reading the first phrase.
Most importantly, the written message you’re trying to send is a tedious one for the user and it’s an artificial one. Whereas, speaking in style (colors, shapes) exposing your version for user behavior (what’s your concept in user interaction) these can be understood even if the natural language the page is written in, it’s not quite to your understanding. It says who you are, what you can do, what you can manage to do, finally, what’s your compatibility with the one reading the page. It’s transmitting a more natural message.
Even more, text requires so little it levels up message transmitters across globe. What makes a user decide if you are serious with your message is the concern you have with how your things look to him. It’s the same as with an office. You could just use plain primitive tables with plain primitive chairs. Or you could take pride in your possibilities and add some better, more comfortable tables and chairs. And then some more: you’d add some personal signatures, paintings, aquariums, a beautiful secretary maybe! 
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The answer is: because we can easily can. Here and now. Even from old scribes days, they did more than just put some text on their scrolls. They added all kinds of compelling stuff, hard to accomplish with the limited technology of their times, which required extremely hard work.
Why would we deny this, given the technology that we have today? Because we don’t act like one? Because we don’t all focus on just one single vision? Where’s the good in that? Even if we still act like a heard, there’s a time for every heard to move to greener pastures. Move from those arid text alone plains to a more enticing scenery. For the joy alone, if not for anything else.