Okay, but why all of that matters? Simply, because of user experience of your web page. Now as we build single page applications, with aim of being available everywhere (responsive ui) it’s becoming important to make them as lightweight as possible, and that usually mean do as small amount of remote requests as possible (concatenating files), making the requested files as small as possible (minification) and much more. Actually on top of that, chrome development tools audit is your friend teaching about all the things you should care about.
And you know what? Besides of learning about the concepts, with grunt you can automate everything easily making it part of your daily build process.
Btw, why I chosen grunt, not gulp? I really like the way of how gulp express configuration, but it looks like it’s not mature enough to have ready to go solution for everything I need. I spent lot of time to look for something, when for grunt it was ready to take (plugins).