During the formation of RBC's, most of the organelles including nucleus is broken down or expelled or reduced. This helps them to reduce their size. Small size is important for RBC to pass through small capillaries. read more
This allows the red blood cell to contain more hemoglobin and, therefore, carry more oxygen molecules. It also allows the cell to have its own distinctive bivalve shape, which promotes diffusion. This form would be impossible if the cell had a nucleus. read more