How Fireworks Work

When that first energy release occurs, conventionally known as the lift charge, it has two important effects

The lift charge imparts an impulse, causing an acceleration, to the remainder of the firework, which includes the other three components.

This upwards acceleration needs to give your firework the right upwards velocity to get it to a safe height for the explosion, and the fuse needs to be timed appropriately to detonate at the peak launch height.