Bio media is the designated layer where bacteria is allowed to thrive and build up over time. These bacteria breakdown organic waste and also oxidizes harmful ammonia into relatively benign nitrates. This layer should not be disturbed as it takes a long time for bacteria to colonize filter media.
Thus the bio media layer should be shielded from debris so that it does not clog up - bio media requires good flow through it for aerobic bacteria action to be effective. A sponge media layer usually acts as the shield, capturing coarse particles that might otherwise easily clog up the bio-media and can be washed/replaced often without disturbing the bacteria in the bio media.
Both sponge and ceramic media can play the role of bio-media for bio-filtration. The main advantage of ceramic media is its durability and that it does not collapse. Specially designed ceramic media can provide regions for anaerobic bacteria action - which allows for reduction/removal of nitrates. Sponge media is significantly cheaper, lighter as a whole. Very fine filter floss/sponge can be used to capture fine particles in the water to give high water clarity.
A basic filter layout would be as such:
There can be many more intermediate layers in between in many setups, as people choose layers that suit their particular setup.
Purigen, if used, should be placed at the end of the chamber to capture the finest particles before water re-enters the tank. It needs to be bleached regularly to recharge it.
If the concern is water polishing, rather than just use one layer of coarse filter sponge, a medium grade filter sponge is placed after the coarse sponge. This captures smaller particles and prevents particulate matter from clogging up the bio-media/fine filter floss.
This is the layout that I use in most of my tanks. I use filters that have fine filter sponges as a pre-filter; these pre-filters are great at removing debris, which allows the entire main chamber (as above) to remain untouched for months (about 4 to 6 months). Occasionally I use purigen in my tanks as well.