While Satoru was "betting time" asleep in his coma, what became of the lives his childhood friends lived without him? This special extra volume of Erased checks in on the lives of Satoru's friends and the bonds they wove thanks to the chances at life his "revival" gave them.
Source: Yen Press
These chapters don't make any sense without the main series, but the main series doesn't feel any worse off by lacking these chapters. In other words, this is pandering to the fans. The synopsis claims that these chapters are filling in the gap during the years when Satoru was in a coma, but three out of the five chapters don't even take place during those years--they are just retelling events from the main series from different perspectives. And even the ones which do take place during those years (ch. 1 and 3) don't really add much new information. In fact, other than chapter two, I didn't find any of these chapters engaging. They didn't make me feel like I understood the characters on a deeper level or anything; for the most part, they were just making explicit things which were already understood but left implied in the main series.