Because Godzilla takes place in the 1950s, news can't travel at the speed of light by way of websites or text messages. And since no one has a social media account—on account that there is no social media to have—not everyone can share news simply by being present. Instead, news reporters act as middlemen, going to places where there's news, writing about it, and giving that information to their readers.
Being a back-in-the-day reporter, Hagiwara goes from place to place to gather information to inform the public. Not coincidently, this is the same role his character plays for us, the viewing audience. By going wherever the story needs him, Hagiwara can give us the information we need to understand the story of Godzilla.
Here are a couple of examples of what we're talking about.
When Hagiwara interviews the villagers of Odo Island, he talks to Masaji about a giant beast in the ocean. When Masaji tells him this information, we get another clue regarding the mysterious ship disappearances. We similarly latch on to the name "Godzilla" because of Hagiwara's discussion with the old fisherman.
Heck, even when Hagiwara doesn't get the scoop, we get the information. Hagiwara tries to interview Serizawa regarding his research, but the scientist denies any knowledge of work related to Godzilla. When Hagiwara leaves, Serizawa confides in Emiko about his Oxygen Destroyer technology. But if it weren't for Hagiwara, Emiko wouldn't have been there to receive the information and, by proxy, neither would we.