why are sugars attached to polypeptide?

Glycoproteins are usually receptor molecules. The Carbohydrate attached to the protein enables other molecules such as nutrients or growth factors to recognize the protein by having specific shapes that can fit those molecules. Once the molecules attach to the receptor proteins, a transduction pathway leads to a particular cellular response.

Sugars are attached to polypeptides for many reasons. Some assist in protein folding - holding part of the protein out (and not buried) so that the protein will adopt an active confirmation. Some sugar molecules act in receptor-ligand interactions, and they are most important in antibody development. Also, they can confer some cells an identity of sorts, showing that the cell is native to the body. Some other cells use these glycroproteins as a coat to avoid immune system responses. Glycoproteins are found in the egg (oocyte), and are important in sperm-egg interactions. Sugar molecules on proteins can also increase their viscosity by allowing them to bind water (egg white or serum albumin is a good example of this). This can also sequester water away from hydrophobic areas (possibly allowing lipid binding) and provide chemical and thermal stability.