Explain why cracking is necessary in petroleum refining?

Cracking is the process of splitting long chain hydrocarbons into shorter chains. The resulting short chains have a lower boiling point due to weaker intermolecular van der waals forces - this is as a result of them having less electrons and a reduced surface area. A lower boiling point results in them burning better as they must be vaporized before burning making them more favourable as a fuel.