Most Helpful Guy
Likely due to survival thing, babies having more fat gives them a better chance to survive if there is a famine or lack of food, also likley to protect and keep them warm.2
Most Helpful Girl
Emergency energy in case of illness or extreme case scenarios. Infants are born with a store of brown fat, which is a very high energy burning fat that animals typically only use if they go into hibernation or torpor (close to hibernation.) Ever hear of a baby who got really cold and seemed to 'shut down'? They will do this if close to starvation as well. If help is gotten soon enough, the brown fat will save them, bring them back from death. We lose this fat as adults (to some degree), but all infants have it.
Other fats are very important for babies to get through illnesses or sicknesses, when they don't want to or are too weak to eat.4