Potassium dichromate is used for several purposes in pyrotechnics.
In solution, it is used as a surface treatment to suppress the corrosion and reactivity of magnesium; infrequently as an oxidizer; and as a catalyst to aid in the decomposition of potassium perchlorate, often in primes. It does this by lowering the activation energy of the chemical reaction that takes place when the composition burns. That makes the star, comet, etc. ignite at a lower temperature. Potassium dichromate enables the oxidizer to part with its oxygen with a lower input of energy than would otherwise be needed. In other words, it allows the oxidizer to decompose at a lower temperature.