Why should I use v-bind for style - vue.js

Question

I just started learning Vue and I was wondering, why should I use v-bind for style and not write it regularly in html/css file

Answer 1

Let's say you need to create a progress bar that is not static. You will then need to update the style attribute width for-example.
To accomplish this, we need to programatically edit the width of the element. We 'cannot' to this in plain css, therefore the :style attribute comes in handy.
Let's create an example:
Codepen
HTML
<div id="vue">
<div class="progress-bar">
<div :style="{'width':progress + '%'}" class="progress" />
</div>
<button #click="fakeProgress">Init fake progress</button>
</div>
Css;
.progress-bar, .progress {
border-radius: 20px;
height: 20px;
}
.progress-bar {
width: 250px;
background-color: gray;
}
.progress {
background-color: blue;
width: 0;
transition: all 1s ease;
}
Javascript
new Vue({
el: '#vue',
data: {
progress: 0
},
methods: {
fakeProgress() {
let progress = setInterval(() => {
if(this.progress == 100) {
clearInterval(progress)
} else {
this.progress += 1;
}
}, 50)
}
}
})
As you see here, we bind the progress data attribute to the width value on the fake progress bar. This is just a simple example, but I hope this makes you see its potential. (You could achieve this same effect using the <progress> tag, but that would ruin the explanation.
EDIT; Also want to point out that you are supposed to write all your css as normal as you point out in your question. However, :style is used in cases that you cannot normally use css for. Like the example above where we need css to change from a variable.