Watch the D open up online guitar lesson by Jeff Scheetz from 50 Blues Rock Rhythms You MUST Know

Using a drone note, like in this case the open D string, can be a cool way to tie a riff together. While we are moving on the G string, that open D keeps the foundation. This is heard in many songs (think Collective Soul) and can be used in a rock context that is heavy, or more "indie" feeling.