Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules. There is more demand for shorter alkane molecules and alkenes than for many of the longer chains formed during fractional distillation. This is where cracking comes in.
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them.
Cracking produces a mixture of smaller alkanes and alkenes. This helps to meet the demand for the more useful fractions and to increase profit.