Why are sugars like mannose expressed on the outside of eukaryotic cell membranes?
The “positive aspect” or reasons why the surfaces of eukaryotic cells contain sugars are various. Major ones appear to be cell adhesion and cell–cell interaction.

In fact, the variation of the sugars in certain cell-surface antigens (blood group antigens and histocompatibility antigens may represent the kind of evolutionary response to infectious agents mentioned in the question. The situation with the major histocompatibility antigen is complex, but variation in blood group antigens is thought to be a response to the malaria parasite — reviewed by Cooling.