N-linked glycans have intrinsic and extrinsic functions.
Within the immune system, the N-linked glycans on an immune cell's surface will help dictate that migration pattern of the cell, e.g. immune cells that migrate to the skin have specific glycosylations that favor homing to that site. The glycosylation patterns on the various immunoglobulins including IgE, IgM, IgD, IgA, and IgG bestow them with unique effector functions by altering their affinities for Fc and other immune receptors.  Glycans may also be involved in "self" and "non self" discrimination, which may be relevant to the pathophysiology of various autoimmune diseases.
In some cases, interaction between the N-glycan and the protein stabilizes the protein through complex electronic effects.