In any organism, all the proteins or enzymes are not required always, which means all the genes are required to express at a given time.
Further in multi cellular organisms different cells of the same individual organism differ in their structures and functions though all these cells are derived from a single cell, the zygote, and thus contain the same set of genetic complements. For example, the liver cells and muscle cells though derived from the same zygote and have the same complements of genes; liver cells produce biles while muscle cells cannot. Similarly, the young plants cannot bear flowers and fruits but the matured plant can, though the gene complements are the same in both cases.
This implies that genes are expressed, only whenever and wherever they are required to do so. Xot all genes present in a cell are expressed at the same time; some are expressed and some are switched off. There is some sort of regulation of gene expression so that genes are expressed in appropriate time and also in specific cell types.
This allows the cell to conserve its resource and overcome wastage. However, there are certain genes which are expressed in all cell types continuously, like the genes for respiratory enzymes, RXA polymerase, DXA polymerase etc. These genes are called house keeping genes or constitutive genes.