Cell specialisation and differentiation allows specific genes to be expressed for cells to partake specific functions in an organism. Somatic (or body) cells differentiate into somatic and differentiated cells, where they begin to express the correct gene for that specific function.
For example: a skin cell starts off as a somatic cell. The somatic cell is undifferentiated and doesn't have a role in the body. Once they undergo mitosis and differentiate into its somatic cell (replication) and its differentiated cell, then it will start to express the specific gene. This process is called cell specialisation. Once they express the skin gene, they have a function as a skin cell.