Why do people like the Grateful Dead?

For many music aficionados, listening to the Grateful Dead is enjoyable, based on their improvisational style, and the long jams.  Prior to the Dead, most singers and bands had short three to four minute songs, that were crisp but didn't allow for the musicians in the band to take risks.  They were just playing from the songbook and the notes written down.  The Dead allowed for a more fluid approach, and provided the musicians with opportunities to go off script, and just play for the love of creating music.    The Jam genre they created led to many other bands performing in a similar style and openness, including Phish, Dave Matthews band, Widespread Panic, Blues Travelers and many more.  The Dead(as they are often called) started the trend though, and will be forever memorialized because of that.