Why would septic system have 2 tanks and a pump?

Many septic systems are 'passive'. That is, the final waste water runs by gravity to a field downhill. However, in the powered system there is a 'settling' tank, for the solids to settle and be aerated or subject to microbial action. This tank has a drain near the top to allow 'black water' to trickle into the 'pump tank' . A pump is in this final chamber, switched by a float. When the water gets to a desired level, the float switch operates and turns the pump on, as water drops to an acceptable level (usually after 50-100 gallons have been pumped out) the pump is automatically switched off by the float and water once again trickles in from the main settling tank.