A find like this can go a long way to explain what a culture was like at the time this body was set there.
Funeral rites and sacrifice areas are rich with clues to the ways of a culture. Religious burials have been seen all over the world. Tombs and crypts can give you a pattern of religious movements in the past. Even cultures that were nomadic can be tracked in this way.
Much data can be gained by comparisons of these burial rights. We learn by seeing what they have in common, as well as what they do different.
The tombs can be an example of what these people thought were important. It can show you how evolved the culture was with the writings in the general area. It can show you how they dressed, what their family structure was like and how they viewed death.
In the case of Tullund Man it gives a picture of the burial rights of this culture with a lot of clarity. A pristine body gives scientists a lot of data to compile.