You should know that fat is a long-term storage form of energy made of glycerol-linked fatty acids composed of hydrocarbon chains linked to acyl groups... as opposed to glucose/sugars or amino acids/proteins or glycogen/starch (and in which kingdoms), for example.
Think about how plants store their energy and how animals store their energy.
Would a bear who hibernate need more fat, or would a hummingbird that rapidly metabolize all available energy need more fat? Etc.