These proteins must act together so that the dance of the chromosomes at mitosis is coordinated with cellular division. At another time in its life, that same cell may have decided not to divide but instead to move from place to place. In other words, cells need to make different structural proteins and enzymes in response to different needs, and these components must interact properly together. Since proteins are specified by genes, each time a cell needs to make a different protein, it may transcribe different genes. We can pose the last question in a different way.
All cells have some components in common.
But multicellular organisms possess different cells that have different specialized functions. Skin cells have to be flat to cover a wide area so as to protect the organism from outside insults. Heart cells must be able to contract in a regular rhythm in order to pump blood through the body. Lens cells in the eye must be clear so that light rays can be efficiently focused on the retina. Nerve cells must be long and cable-like in order to pass messages from one place to another.