A mixture is a combination of different substances, so we need to separate the useful components from the non -useful components.
Also, we need to remove the harmful and undesirable components from the mixture which can be done by separation. We often extract crude oil from the ocean which contains very useful products with highly corrosive and harmful components such as sulphur or acids, which must be removed. By separating crude oil, useful substances like LPG, petrol, kerosene, diesel, etc. can be obtained.