WARNING: The following contains spoilers for One Piece Chapter #1028, "Brachiosnakeus," by Eiichiro Oda, Stephen Paul, and Vanessa Satone, available in English from Viz Media.
When Nico Robin first joined the Straw Hats, fans had no idea how infamous she was in the world of One Piece. It was when Rob Lucci and the rest of the former members of CP9 were introduced in the Water 7 Arc that things started to become clear. Lucci and his crew managed to get Robin to give herself up, but the Straw Hats eventually came to her rescue and defeated each CP9 member.
Fans might have thought that Lucci moved on from trying to capture Robin after the Enies Lobby arc. After all, in-universe it's been at least two years since CP9's embarrassing defeat. However, Chapter #1028 shows that's not the case. The World Government, and Lucci in particular, are apparently still obsessed with capturing the Straw Hats' archaeologist.
Chapter #1028 shows Lucci speaking to the CP0 members who are currently present in Wano. They discussed whether Kaido being defeated is an actual possibility. They can't see that happening, but Lucci revealed that on the unlikely chance that Kaido loses the battle, the Land of Wano will be placed under the government's direct control. He further emphasized that regardless of which way the battle goes, they must capture Nico Robin.
Nico Robin has been on the run since she was eight years old when the World Government destroyed her home island, Ohara. She spent 20 years evading capture by the government, despite people she thought were helping her, ending up trying to turn her in. The World Government branded her the Demon of Ohara and led people to believe she posed a major threat to the world.
The real reason the World Government is so obsessed with capturing Robin, of course, is because she's the only living person who can read Poneglyhps. This essentially makes Robin the single most important character to the overarching plot of One Piece. For reasons yet to be revealed, the government doesn't want anyone to decipher the information on the Poneglyphs.
Historical Poneglyphs reveal important pieces of ancient history that have been lost to the modern world. This ancient history includes information regarding the Ancient Weapons and the period known as the Void Century, the latter of which the World Government has forbidden research. It's said that by learning of the Void Century, one becomes knowledgeable about the true history of the world.
The reason the World Government had Ohara destroyed was that the island's scholars were researching the Void Century. Robin was the sole survivor and the World Government has been hunting her down ever since. They told the public a fabricated story that the Ohara scholars were searching for Ancient Weapons in order to destroy the world. This was just an excuse to cover up the real reason, in order to avoid the public becoming suspicious of the missing pieces of history.
There are two other types of Poneglyphs aside from the historical ones: Instructional Poneglyphs which provide hints about where to find the Historical Poneglyphs and Road Poneglyphs which allow one to find the last island at the end of the New World. In other words, Poneglyphs are also the key to locating the coveted One Piece treasure that Gol D. Roger left behind.
As the only living person who's able to read the Poneglyphs, Nico Robin is the single most important character in the world of One Piece, who isn't already dead. She is the key to the series' main protagonist achieving his ultimate goal of becoming the Pirate King as well as finding out what exactly the Void Century was, which makes her the only person who can reveal why the World Government so desperately wants it to remain a forgotten period of history.