It goes without saying that one of the many reasons as to why Oda's work is so successful is because the cast in One Piece is nothing short of phenomenal. Not only do the fans follow the journey of Luffy and the Straw Hat Pirates but they also get to experience the journeys of some of the most incredible characters in the story, such as Kozuki Oden, through either flashbacks and, sometimes, the unique Cover Stories that Oda incorporates in his manga.
As the name suggests, Cover Stories are additional stories from the world of One Piece that Oda tells through the chapter covers. Since a majority of the main story focuses on the Straw Hat Pirates and their adventures, Oda usually chooses to tell the stories of some side characters through these pages where fans get to see what they're up to and how they've changed since the last time they were in the story. Although supplemental to the manga, all Cover Stories are fully canon, meaning they are included as a part of the whole story.
When it comes to the format of the Cover Stories, it appears Oda isn't very formal with them as he simply draws an illustration without any paneling and gives context to the illustration at the bottom of the page. Just like the manga arc, every Cover Story has its own title and with each chapter, fans get a volume of the Cover Story, which can span over multiple chapters.
Cover Stories are quite easy for fans to overlook, especially for the anime watchers, some of whom don't read the manga at all. At times, even fans of the manga tend to not pay too much attention to what's going on in the Cover Stories, however, on several occasions, Oda has tied these stories into the main storyline, which just goes to show how important they truly are.
A great example of this is the famous Cover Story on Portgas D. Ace, the Second Division Commander of the Whitebeard Pirates and Luffy's brother who, after the death of Thatch, left on a mission to find Blackbeard. As fans know, Ace eventually meets Blackbeard on Banaro Island in the Grand Line, however, before getting there, he has quite an adventure which is told through the "Ace's Great Blackbeard Search" Cover Story.
Understandably, Ace has no idea of where Blackbeard is when he sets out on his journey, and eventually, he comes across a Doctor named Black Beard. Not realizing he got the wrong guy, Ace gets into trouble with the townsfolk and ends up being saved by a girl named Moda. To pay his debt to her, Ace then goes to the Marine G-2 base where he delivers her letter to Vice Admiral Comil, and before leaving the base, he gets information on where Blackbeard was last seen, and with that Ace sets his path towards Banaro Island.
Interestingly, Moda, who saved Ace, is later introduced in the manga when Pinkbeard, one of Blackbeard's underlings attacks her town. Thankfully for her, the Revolutionary Army Commanders make their debut and save the day. Although Moda herself isn't all that important to the story, the fact that she receives her own little story which is then used to build on in the manga just makes One Piece all the richer.
Some Cover Stories simply show the reactions of characters to the Straw Hat Pirates, such as the famous "From The Decks of the World," Cover Story, which was the very first one that Oda illustrated after the two-year timeskip. As the Straw Hat Pirates reunited on Sabaody Archipelago, the world came to realize that the crew are back together, thanks to the News Coo spreading it all across the globe. The likes of Dadan Family, Makino, Chocho, and many others who were connected to the Straw Hat Pirates' became a part of this story and thanks to this, fans get to see what glimpses of these characters who usually don't appear in the manga due to Luffy's journey being far away from them.
Other Cover Stories show how the villains of the story are doing and how they've fared since their defeat to the Straw Hat Pirates which, once again, goes to highlight their importance to the story. The "CP9's Independent Report" is a great example of this type of Cover Story as it showcases how the group escape from Enies Lobby and then tended to Lucci's wounds following his battle with Luffy. It also adds more depth to the CP9 and the World Government plotline by showcasing the rise of a new generation of Cipher Pol and, more importantly, showing to the fans that Spandam and Spandine are on to their clever tricks once again.
It is quite clear that Oda's Cover Stories are very important to the plot of One Piece and if one wishes to absorb the story in the best possible way, reading the Cover Stories is essential. Not only do they add to the narrative threads that are eventually brought up in the story but they also make the world of One Piece feel alive by showing the fans that there is life outside of the Straw Hat Pirates as well. Above all, Cover Stories are yet another way for Oda to express his love for One Piece, and just to make the story a bit more interesting and the world more alive, he goes the extra mile.