Eliminating bypass flow is important both for the designer of the filter and for the hobbyist trying to maximize the filtration function of the filter. I like to think of it as not allowing the water to "cheat" and get around the media without coming into contact with the surfaces of that media.
Ceramic rings are interesting. Why rings? The answer, in discussions that I've read is that rings act like short pipes, forcing the water to change from one larger general direction of flow into many smaller flows of very different angles. When a fluid makes a sharp turn, it slows down (some of the energy of its molecules is transmitted to the ceramics as they bump and those fluid paricles slow a tiny bit.) Reduced flow makes it easier for debris particles to stay put after they are trapped at a corner between rings.
The larger weight of ceramic rings allows them to sit still and not rattle around in the flow, allowing further particle settling and keeping the filter quiet. Larger diameter rings and center holes allow larger particle trapping spaces, differentiating the rings from the smaller interparticle spaces of media beds that follow.
The reason the rings are made of rough ceramic is that this has been found to be one of the ideal surfaces for autotrophic bacteria and the biofilms they construct. Ceramic, unlike carbon or filter floss, will not easily break into smaller particles that can be washed away in the flow or when it is rinsed.
That's the theory, what about the actual implementations we see in various products? Well, lets take the rings pictured in this thread. To my eye, these rings have a pretty large diameter but quite small holes for the "pipe." More of the mass of the ceramic material is trapped inside the solid space of an individual ring, where it can't offer any surface area or do any good. If the diameter of the inner pipe were larger, it would have more surface area and it would differentiate itself more from the smaller sized media materials that should come next in the water flow (the next tray or bed of media, such as ceramic gravel.)
In this case I don't think its a big deal, but its interesting to speculate whether sometimes the ideal theory is lost in implementation because of some less labor intensive manufacturing process or perhaps that the theory was not fully understood by the particular manufacturer.
~~waterdrop~~
