At first, picric acid was used as a booster to detonate TNT, though it was superseded due to the inherent danger of picrate formation.
Tetryl replaced picric acid because it is more stable, and was once a very popular chemical for booster charges, particularly during World War II.
When encountered in connection with artillery shells or air dropped bombs, a booster charge is sometimes referred to as the "gaine", from French: gaine-relais.
At a purely technical level, a sufficiently large detonator would initiate high explosives without the need for a booster charge.
Therefore, minimising the amount of primary explosive that users must store or carry greatly reduces the likelihood of serious accidents.