why is it that you put the javascript enqueue without the jquery dependency in the header?

Here's a more clear answer... deleted my previous one.

Wordpress ships with the most recent version of jQuery at the time of the WP release.

In the video (at 4:00), Zac didn't create a specific enqueue call for jQuery because he instead set it as a dependency of another enqueue call. Foundation.js requires jQuery to run, so Zac put jQuery as a dependeny of foundation.js, which tells WP to load jQuery before it loads foundation.js. This is cleaner and less code, but you could still create an enqueue call for jQuery if you weren't setting it as a dependency of another enqueue call.

Also, he set the last parameter to true, which tells WP to insert this script into the footer instead of the head. The benefit of loading scripts in the footer is page load time. If all of your scripts load in the head then the page waits for those to complete before it renders your page html. This can cause lag or slow page loading. So by calling scripts in the footer that are non-essential to rendering your page, your page loads faster. In the case of modernizr.js, it affects how your page renders, so you would want that to load in the head before the page renders.

I hope that's more clear.