akamel

what are they ? when and why are they needed

Lucy

It's part of a filter shaped that's shaped like a wheel.
When water goes up the intake and out the outtake, it makes the wheel spin.
When it turns it mixes with water and air allowing beneficial bacteria to grow on it.
If you don't see one in your filter, you probably have a coarse sponge and cartridge or just a cartridge.