What is the reason for using copper thermite? Why go to the trouble, when other ignition methods are less hassle?
As a general rule, larger solid motors tend to take longer to fully ignite and come up to pressure than smaller motors. If you're only lighting a single motor, then a second or two of chuffing on the pad is probably not a big deal. However, if you're lighting a cluster of motors, a safe flight may depend upon ensuring that all of the motors light simultaneously.
Also, when a motor chuffs and struggles to come fully up to pressure, it is burning propellant that could be put to better use (flying the rocket), and it may also be burning some of the delay charge, resulting in a premature ejection of the parachute.
Copper thermite, properly used, allows you to acheive "instant on" ignition of solid motors. The thermite lights easily from an electric match, and instantly throws a fine mist of molten copper metal at the walls of the motor core. The hot metal droplets ignite the entire inside surface of the motor simultaneously, resulting in an "instant on" chuff-free motor ignition.