In a refinery, crude oil is broken down into its various components in distillation columns. In the subsequent cracking process, the heavy components, which are long-chain hydrocarbons, are broken up into shorter-chain molecules. This is necessary because the market demands more short-chain hydrocarbons (gasoline, diesel, light heating oil) than crude oil contains, whereas long-chain hydrocarbons (heavy heating oil) are being used less and less often. The cracking process generates alkanes and alkenes, which the chemical industry uses as raw materials.
Source: Association of the German Petroleum Industry