I haven't gone into cab design in any depth, because it's such a complex subject, I could (and may) do another whole site, just on cabinet design.
BUSTED : The most common rig-damage you'll come across is busted speakers. Here's how they happen and how to avoid it.
The vast majority
of decent cabs use horn loading to help control directivity, linearity and
general niceness (my way of saying other, bastard complex reasons).
Cheapy speaker boxes, like most home speakers, use direct radiation, with the cone bolted to the front of the box. This is fine, until you start trying to have high-power speakers, where sound flays out everywhere and box resonance and design starts becoming more important than the quality of the driver itself. Basically, direct radiation is shite.
Horns, however, are cool. The
easiest example is the high-freq. horn, which directs the sound. These are
common, because they're small and easy to make.
Essentially, the horn mechanically matches the driver to the air, compensating for failings, increasing directivity. Thus they are found on mid and bass. Mid is obviously helpful in establishing a stereo image and penetrating distance.
While you may well say "bass goes everywhere anyway" that's the whole point; mostly bass gets lost all over the place, whereas horns help you get more of your hard-earned energy directed at the audience where you want it.