Why are only some genes expressed?
Gene expression is defined as the process by which gene is expressed or used for production of functional protein or gene product.
All cells have same in your body has same set of genes, but when cells divide and grow then different cell types are formed that results in different genes expression.
For example, liver cell perform different biochemical duties from nerve cell which means different proteins are made as different genes are expressed. This is because different gene sets of must be turned on and off which means expressed or not in the various cell types.
An organism require different proteins at different places in body and at different times. Genes will be turned on and off in cells according to their need. For example liver cell expresses particular gene and rest of the genes are not expressed, or turned off as they are not needed in liver cells.