Cells are of different shape, size and structure according to the function they need to perform. So, if all the cells become of same size, shape and structure, it will be very difficult for multicellular organisms including humans to adapt better to the environment and survive. Also, the division of labour among the cells will be lost thus leading to difficulties in survival.
Nerve cells are usually branched and very long as they need more surface area to transmit the signals. Due to their long size they can easily carry messages in each and every portion around the body.
In mammals, after maturation, the nucleus disappears from the RBC to provide adequate space for hemoglobin to carry oxygen. Nucleus is responsible for cell division so the blood cells don't divide, they are produced by other cells within bone marrow. This is an adaptation which allows the red blood cell to carry more oxygen.