The second act is usually the longest, so there’s a clear midpoint. Because the second act is so long, there is a risk that the readers will grow bored before the climax, which is where everything goes to hell. So the writers strive to keep things interesting.
Sometimes this is marked by the introduction of a new character. In Rambo: First Blood, that would be the arrival of John Rambo’s former instructor. It could also feature a major revelation about a character. At the halfway point of Iron Man, it was revealed that the hero’s mentor was in league with terrorists. This serves a narrative purpose by ensuring that the viewer remains interested in the ongoing storyline, building suspense through dramatic irony. The midpoint can also be a meeting between two major characters who are on the same side. It’s about halfway through Star Wars that Luke Skywalker rescues Leia.
Some would argue that because the second act is so long and usually has a clear midpoint, we should actually be talking about four act structure rather than three act structure. Comic book editor Andy Schmidt suggested that the ideal length for a comic book story is four issues, with the second and third issues covering the so-called second act. Usually the midpoint doesn’t feature a reversal of fortunes, so it is different from what we think of as the Act 1 and Act 2 turning points. Instead it features an indication that the quest is suddenly more dangerous and difficult. But it’s not the moment at which the hero is in any danger of changing his mind.