I have a theory. Sound ports are awesome. I have another theory. Sound ports allow guitar tops to vibrate more freely and with less energy loss. The first theory may be a bit hard to disprove seeing as sound ports are awesome. As for the second theory, well, let me explain...
The primary function of sound ports is to make it easier for
the player to hear without any noticeable difference out front. The secondary benefit, and potentially more important, is the increased freedom it gives the top to vibrate. How does it do this? Imagine pouring water out of a bottle.... the water glug-glug-glugs as it fights the vacuum created inside the bottle. What happens when you poke a hole in the side of the bottle? Air flows in and the water pours smoothly out. Now think about a guitar top vibrating after you pluck a string. As the top pumps up and down there will be some resistance as air is pulled or pushed through the soundhole. Poke a hole in the side and voila! Smoother airflow and less energy wasted!
There is a school of thought amongst guitar makers concerning how the air inside the guitar interacts with the top and back. Through bracing and careful tuning many builders attempt to couple the top and back together as one system to maximize efficiency. In theory I think this would work, but in practice people like to play standing up, leaning back on a couch, on stumps around a fire, and basically all sorts of positions that introduce a giant damper (ie. your belly) on that precision tuned back. For this reason I like to make my backs stiff and focus on the tops. And what can I do to eke out as much energy from the tops as possible? Sound ports!