Your tank is receiving water - and dust - from your roof, so there will be a build up of sediment over time.  The outlets are positioned to allow a certain amount to settle in the bottom, which is a standard practice with rainwater tanks.

Prevention is the best cure - a range of accessories can be used to stop debris entering the tank.  Your distributor will be able to advise you, a range of tank accessories can be seen at the Rain Harvesting website.

It is not imperative to the functioning of the tank, but if you wish to clear the sediment you can flush it out using a hose.  Wait until the tank is empty and open up one or more of the lower outlets.  Remove the inlet strainer and use a hose to flush out any sediment.  Do NOT climb into the tank at any time.