Your sample is ignited remotely once the flask is filled with oxygen and sealed. No need for the cumbersome task of lighting the sample manually, and then quickly inserting the sample holder into the flask and then tightening down the flask top. there is no possibility of sample loss.
The Schöniger flask method is a well proven technique for the combustion and then subsequent analysis of a range of elements including Chlorine, Bromine, Iodine, Sulfur, Phosphorus and a number of metals.