If the tank is too full of solids, they can either be washed out with the wastewater, clogging the outlet, or back up into your pipes. If this does not get fixed you are at a much greater risk of having sewage come up through the drains. 

By regularly pumping your system you prevent any small issues with your septic system from growing into a major problem. Depending on certain factors, how often a septic system should be pumped will vary but it is generally recommended to pump every 3 -5 years. 

If you have not pumped your septic tank in the last three years, now is the time to have it done because your septic system can leak damaging your properties soil and water supply.