HTML5 Boilerplate is basically just that, a template or a basis on which to build an HTML page. It doesn't have much mark up or components out of the box. In other words, it doesn't provide all the components and tools needed to build a UI, it contains best practices and a generall template on which to build your UI.
Modernizr checks browser support for HTML5, CSS3, as well as a few other miscellaneous elements and allows for easily writing fallbacks if they're not supported.
To help out newcommers to either programming in general or HTML5 Boilerplate specifically, the boilerplate includes in-depth docs that help understand how things work and why certain choices are made. The documentation is highly comprehensive going even as far as to describe every single line of normalize.css that the H5BP includes.
HTML5 Boilerplate theme doesn't only provide a base template for building your next modern site. It also has performance optimization features built into it which include stuff like html + js minification , image size reduction as well as css minification. This link here describes how to optimize your shiny new html5 site using these built in features.
With optional support for XHR, we can use techniques such as XHR eval and XHR injections to optimize the loading of external scripts and significantly reduce page load times. Alongside XHR, built-in support for Flash means that you can use your old swf/flash content on your website, where you feel you're not ready for HTML5 yet.
Initializr allows you to "fine-tune" your template by allowing you to take the good stuff from both HTML5BP and Twitter Bootstrap and download a mashed up basic template, which you can begin development on.
Initializr is very well documented, even though it itself being a pretty easy to understand and use tool. The documentations are available in different languages as well, available via the International Guides section on the website.
With initializr, very little setup is needed. All you need to do to be ready for development is provide your favorite template maker (HTML5BP or Bootstrap).
Some developers much prefer Foundation over Bootstrap due to the flexible grid and large selection of widgets. However, Initializr supports only Bootstrap.
Automatic automatic image optimization is a pre-built functionality of the web app generator. So that any sites that are made using this tool are perfectly optimized for performance.
As the title suggests, the yeoman watch will auto-compile your JavaScript and Sass scripts for you. Also your code will be run through JsHint (for liniting) to make sure best practices are followed as well.
Since a lot of web designers these days don't use the plain old CSS, instead they use a CSS pre-processor (like Sass or Less). Thanks to Sass support web designers can easily add GWK to their workflows.
Since the base skeleton provided by the Web Starter kit is authored by Google itself, it's pretty much understood that the template is optimized to the max, for performance. If you test a site (on Google PageSpeed Insights) built by the GWK you can be sure that it'll always achieve a very high score.
Since most of the websites that are currently being built (or any website that was built after 2012) are responsive, a 'scaffold-ed' website skeleton without any responsiveness would be pretty useless.
In case you want to try out your shiny new site on your mobile phone or another PC. Just type in the command gulp serve and load up the provided server address from another device to see how your website looks and performs.
If you use the Gulp build system and are familiar with its environment, you can directly use Web Starter Kit's Gulp build script to build your next GWK based project.
There's a Yeoman generator for Google's Web Starter Kit available. To install (requires Node, Ruby, Gulp, and Sass):
sudo npm install -g yo generator-web-starter-kit
cd project-root-directory/
yo web-starter-kit
npm install
gulp serve