The messenger may seem like a tiny, unimportant role, but going as far back as the beginnings of tragedy in Greece, messengers have been crucial. Why? Well, the Greeks never got your creative writing teacher's memo about showing rather than telling, because in Greek tragedy all major action or violence occurs off-stage.
What does that mean? Messengers have the big responsibility of updating main characters (and the audience) of what big thing just happened. And that's exactly what our messenger in Samson does, relating with an impressive amount of detail the suicide attack on the Philistine theater.
While the messenger in Samson doesn't have a name, this doesn't mean we don't know a few things about his identity. We know he's an Israelite, which is important for a couple of reasons:
First, it means he's sympathetic to Samson's cause and obviously relates his story to communicate that, so it's fair to say we're not getting an unbiased account. Second, it means that when the messenger witnessed Samson's act, he was in the open-air, cheap seats and so wasn't injured or killed by the collapse of the theater. (Because then, you know, he wouldn't really be able to tell us about it.)
What does this mean for the larger picture? Well, it might be one way of suggesting that things are looking up for the Israelites—and that, once again, an apparent weakness (poverty, being common) is actually a strength. It means you survive.