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 n...
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
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.
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.