Why is it that only part of the genes are working in various cells of an organism?

In all somatic cells of a healthy organism, the chromosome set is the same. This means that the set of encoded information is identical. Moreover, each cell contains all the genetic information of the organism. But all the genetic information is not used by every cell. Each cell uses only that part of the genetic information that is necessary for this cell to perform its functions. Over time, the set of genes used by a cell can change depending on the needs of the body at a given point in time.