Answer:
Small saccharides are normally not immunogenic, therefore the conjugation of small saccharides to a carrier protein increases their immunogenicity. Hence, they are usually linked to a protein such that the structural integrity of the saccharide is maintained.
In addition, the specific chemical modification of a protein or proteins with sugar or saccharide moieties can enable the study of fundamentals in physiology and biochemistry. These type of studies may allow for the development and production of next-generation therapeutics or vaccines.