Knowledgeable gamblers are those with experience who start a game, in this example a sports bet. They have gained knowledge in the past and already have a few starting points before starting a game. They may even have built up preferences, for example which bets they want to participate in, at which margin etc.
Experienced players also know that less skilled gamblers struggle with emotions. Emotional players know, for example, that their club has little chance of winning a match, but betting on their loss feels like betrayal to them.
Bookmakers know that experienced players usually bet first. The much larger group of emotional gamblers bets much later, sometimes even just before closing. However, the median of their bet is almost always close to the outcome of a match, the end result. And the bookmaker had already bet on that (well, calculated with his computers). The experienced player can also take advantage of it before the closure.