Why is cracking needed?

Generally speaking most crude oils contain far more of the less useful larger hydrocarbons and fewer of the more useful small hydrocarbons. Cracking produces both smaller alkanes (useful as fuels such as petrol and diesel) as well as alkenes (e.g. ethene and propene) which are key starting materials for the manufacture of polymers (plastics) in particular.

Cracking is needed to break down larger hydrocarbons (alkanes) into smaller ones that can be used. These are usually alkenes and alkanes. these are called fractions. These fractions are useful for the petrochemical industry.