The beneficial bacteria colonies function best when the water flow is slow and the media (and the colonies) is left undisturbed. Hence the dedicated bio-filter. Separate mechanical filtration takes care of the large particulates and it is serviced frequently not only to remove the crud to prevent it breaking down into dissolved organic compounds, but also to prevent BB developing here instead of the bio-filter. The inlet tube for the bio-filter is positioned higher in the tank, which prevents most detritus from entering...still, a thin pad prevents any particulates from clogging the bio-media.
So with a larger volume of bio-media in a dedicated filter with slower water flow, we leverage bio-filtration to better purify the water.