According to DW-World.De, researchers have discovered what the Nebra disc was used for:

The 3,600 year old disc was unearthed in Germany in 1999. Scholars have determined that the disc was a device used to keep lunar and solar calendars aligned. The disc was used to determine when to add a 13th month to resynchronize the different calendar standards as they diverged over time.
As the article notes, that’s a pretty amazing find since it puts sophisticated astronomical knowledge into the hands and minds of bronze age civilization. Even then, apparently the makers of the disc weren’t able to hold on to the knowledge very long. According to DW-World.De,
But the German researchers also discovered that in the 400 years that the disc was in use, its status had evolved. The perforations on the edge of the object as well as a ship that was later added to the map suggest that the knowledge about the lunar calendar’s shortage of days was lost along the way.
“That means, that in the end the disk became a cult object,” [archaeologist Harald] Meller said.
Source:
Bronze Age Sky Disc Deciphered. DW-World.De, February 27, 2006.