As a resident of a college town in Iowa, of presidential primary caucus fame, I have attended more than my fair share of political rallies. From my perspective as an audience member, there are a few things going on with the folks seated on the stage (obviously this answer is US-centric):
First, remember that the live audience is often the main target of a rally, even if a soundbite or photo op might make it into the wider media. Although a few events are televised in their entirety, most of them are not. And even most of the events that are televised rely on an enthusiastic live audience. So some of the staging is for the benefit of the people who are actually present at the event.
- The people selected to sit behind the speaker are generally drawn from the front of the line of spectators.
- This provides a bit of an incentive to show up early, which helps with the whole sense of urgency and importance that these events want to generate.
- It also helps the events to feel more democratic (little d), given that there is also usually a VIP section whose members generally get better views, more access to the candidate/speaker, etc.
- Hopefully (for the rally organizers) this also puts some of the most enthusiastic supporters front and center.
- Having audience all the way around the speaker helps create a greater sense of energy and solidarity.
- Between the audience and the speaker: the speaker is literally surrounded by supporters. If you've ever attended a theatre-in-the-round, think about how much more immediate the action seems than with the traditional separation between stage and audience of a proscenium theatre.
- Among the audience: everywhere an audience member looks they see (theoretically) like-minded individuals. If the on-stage audience members are responding the way the rally organizers hope, some of the "approving nod" effect you posit might work here, similar to a sitcom laugh-track.
- One caveat: I'm not sure how this carries over to enormous stadium-style rallies, as I haven't attended anything with an audience larger than a couple thousand.
Having said that, there is definitely some staging going on for the benefit of whatever cameras might be present. This is pretty clear as rally organizers will generally do some "grooming" of the on-stage audience before the event starts.
- Even though the on-stage audience is largely determined on a first-come, first-served basis, some folks are a little more likely to be picked (and may even be added later, after most of the on-stage audience has been seated for a while).
- The camera likes kids. Being near the front of the line AND having a kid or two in tow will almost guarantee you a seat on stage, and probably very near to the center of the bleachers.
- Depending on the rally, other demographic issues might lead to folks being shifted to be more or less directly in the camera's viewfinder. At one rally I attended, it seemed that there was some concern about how many guys wearing Cubs hats were on-camera. I have also seen audience members in a certain age group approached to squeeze into the on-stage seating.
- The on-screen audience may be adjusted to be more visually supportive of the cause.
- Rally organizers will often have pre-made signs and other swag (like buttons and stickers) to hand out to audience members; they especially concentrate their efforts on the on-stage audience.
- Folks wearing slogan-emblazoned shirts and hats may be re-arranged to be more concentrated right behind the speaker.