[Alneyan]

Break formation more or less means your ships go back to their ship strategy, and that they do not follow the other ships. Breaking formation has no specific disavantage, so there is no reason not to do it.

Having ships keeping their formation might seem a good idea (such as when attempting to use a Planet Destroyer, while keeping a few escorting ships with PDC), but it simply does not work. In the case of a Planet Destroyer and its escorts, the fleet has the annoying tendency to slowly head for the planet, thus sealing its fate if the planet has weapon platforms.