Why is the Rio Poneglyph so important to Robin? In fact her only reason to go to the end of Grand Line is to find this Poneglyph.
It's important because it's said that it contains information about One Piece history including the Void Century.
The Rio Poneglyph (真の歴史の本文, literally translated as "True Text of History") is the message carried by all the poneglyphs along the Grand Line. It contains the true history of the One Piece world (including the "Void Century" of which research is forbidden). In order to find it, one must take all other passages from the other Poneglyphs with them on their travels, and it is only completed upon reaching the end of the world; in other words, the island Raftel. Gol D. Roger seemed to have accomplished this task, according to Rayleigh, though it is uncertain if the other members of the Roger Pirates (i.e. Shanks, Buggy) know the world's history.
Source: Poneglyph - The One Piece Wiki
Why is it important to Robin?
Nico Robin was born on the island of Ohara , and came from a family of archaeologists. Her mother, Nico Olvia, went out to sea to find the True History. But she along with the archaeologists of Ohara were killed by the World Government through a buster call.
So naturally, Robin became an archaeologist to finish the work started by her people and to know the true events of history, thus exposing the World Government.
Why is the Void Century important? Let's take a look at few events that occurred during the Void Century:
- Birth of the World Government
800 years ago, at the end of the Void Century, the World Government was born and took political control of the entire world, uniting all countries and forming the Council of Kings. For the World Government, the events of the Void Century were better left unknown as information linked to it is considered far too dangerous.
- The Great War
The most important event during the Void Century seems to be the Great War. It ended with the fall of Shandora [City of Gold]
Source for blockquotes: Void Century - The One Piece Wiki (emphasis added)