It's my hope that once you have seen a few examples of Flexbox, you will appreciate its utility and feel empowered to use it. However, manually writing all the necessary code to support each of the different Flexbox specifications is a tough task. Here's an example. I'm going to set three Flexbox related properties and values. Consider this:

.flex {
    display: flex;
    flex: 1;
    justify-content: space-between;
}

That's how the properties and values would look in the most recent syntax. However, if we want support for Android browsers (v4 and below) and IE 10, here is what would actually be needed:

.flex {
    display: -webkit-box;
    display: -webkit-flex;
    display: -ms-flexbox;
    display: flex;
    -webkit-box-flex: 1;
    -webkit-flex: 1;
        -ms-flex: 1;
            flex: 1;
    -webkit-box-pack: justify;
    -webkit-justify-content: space-between;
        -ms-flex-pack: justify;
            justify-content: space-between;
}

It's necessary to write all that because in the last few years, as browsers made experimental versions of new functionality available, they did so with a 'vendor prefix'. Each vendor had their own prefix. For example -ms- for Microsoft, -webkit- for WebKit, -moz- for Mozilla, and so on. For every new feature this meant it was necessary to write multiple versions of the same property; the vendor prefixed versions first, and the official W3C version at the bottom.

The result of this spell in web history is CSS that looks like the previous example. It's the only way to get the feature working across the widest number of devices. Nowadays, vendors rarely add prefixes but for the foreseeable future we must live with the reality of many existing browsers still requiring prefixes to enable certain features. This brings us back to Flexbox, an extreme example of vendor prefixing thanks to not just multiple vendor versions but also different specifications of the feature. And understanding and remembering everything you need to write in the current format and each previous format is not a whole lot of fun.

I don't know about you, but I'd rather spend my time doing something more productive than writing out that little lot each time! In short, if you intend to use Flexbox in anger, take the time to setup an auto-prefixing solution.