Cracking converts longer-chain hydrocarbons into shorter-chain hydrocarbons.Cracking an alkane would give a shorter-chain alkane and an alkene.There is a greater demand for shorter-chain hydrocarbons than longer-chain hydrocarbons, so there is a surplus of longer-chain hydrocarbons.Shorter-chain hydrocarbons make better fuels and can also be used to make substances such as polymers.