Some of the fractions are nearly ready to use. However, other fractions need more treatment.
Some treatments, like cracking, split the hydrocarbons into smaller groups. Others, like reforming and polymerisation, change the structure of the molecules.
Some of the lighter molecules are used as feedstock for the isomeriser and reformer.
Heavier molecules are often fed to the catalytic cracker (to be broken into smaller hydrocarbons). Or are cleaned up in the residfiner before being used as fuel oil.