Feathered flightless birds could save energy by being insulated by feathers, providing more energy to use for things like reproduction or growing larger. This would have contributed to their evolutionary success.
Work Step by Step
Consider what insulation means: saving a resource (energy) which could be used for other purposes. It is like someone giving you a handful of money--that could be used for food--allowing what you yourself earn to go to building a better house or having more furniture.