They stay in the sky longer and shine brighter than ordinary fireworks - so it's only natural to blow up expired sea munitions on New Year's Eve. However, this is prohibited under penalty of law, and for good reason. The chemicals they contain change over time, meaning that the rockets don't stay aloft as long, but burn longer - a fatal combination in case of doubt. Especially as a signal star cannot simply be extinguished with water.
Even the extremely hot smoke pots must not be used on New Year's Eve. You can read all about why this is the case, the risks associated with the improper use of pyrotechnics and how owners can use old muniton sensibly instead in the latest issue of YACHT.