The inclusion of live rock as your primary filtration is central to the success of your tank. However, as is quite often the case, the capabilities of the nitrifying bacteria living among the live rock can become overstretched.
Consider your tank a breeding ground for ammonia/nitrite loving organisms which expel nitrate as an end result. Ammonia (in trace amounts) is introduced into your tank by the addition of fish and food, it gets quickly broken down into trace amounts of nitrites, and then into nitrate.
To a certain extent, live rock can handle nitrate, converting it into nitrogen which is then expelled at the water surface. However, adding more fish introduces more ammonia/nitrite/nitrate.
At a certain point the nitrifying bacteria living in your live rock will become inundated by the bioload imposed on them. Unable to cope, nitrate will begin to steadily rise. This is where additional biological media comes into play.
Bio media provides additional ‘living quarters’ to further increase the population of nitrifying bacteria in your tank, thereby tackling the additional bioload your live rock bacteria is unable to handle alone.