In the old days kilns were fired with wood. This resulted in flying ashes that fell on glazed pieces. To protect the glazed pieces they were put in a closed ceramic pot (saggar) during firing. This method was used in Asia (Japan, China, Korea) but also in Great Britain. The word 'saggar' comes from 'safeguard'. The use of electric kilns made saggars redundant.
I use the saggar in the opposite way. The wood and other materials are put in the saggar together with the work. The saggar is closed to protect the kiln during firing.