Rock and roll isn't all about the amp. The acoustic guitar is definitely preferred in certain situations, whether it's only for parts of a song or for the entire piece. A great unplugged introduction can be more than enough to hook listeners in for an entire track, every bit as much as an electric one.
Starting off with a more raw sound can help a song build up to something more grand, or simply establish the kind of song you're about to get. Often the introduction is basically just how the guitar is played throughout the song before the vocals and the rest of the band have joined in, which is no less beautiful than one more distinct from the rest of the song. In fact, setting the tone is pretty essential; imagine how awkward it would be if "Here Comes the Sun" or "Stairway to Heaven" began without their wordless introductions.