Carbonisation is usually done to increase the carbon content of any material if it is desired in the reaction. The carbonisation of any biomass is necessary as it contains moisture and many volatile components that are not desired and may interfere the reaction and these, volatiles can be removed by carbonisation by breaking the bonds at higher temperatures. The purpose of the carbonisation depends on the application and it also used to valorize few materials like biomass.
Carbonisation is basically enhancing or highlighting the carbon content in the matter and removing all the other volatile content and the the process depends on the application of the material you are intended to develop