Specialized functions of reproduction,respiration digestion circulation,and excretion in higher order organisms call for differentiation.Regulation of gene expression makes differentiation possible.
All multicellular organisms start life as a single cell the zygote.
The zygote carries within itself the complete suite of genes that the organism possesses.
As the zygote divides and the cells multiply they begin to differentiate. Some cells go on to become the central nervous system others the limbs,lungs alimentary canal etc.
This can be possible only when some genes are expressed and others suppressed.
A very good example of why gene expression is necessary,is provided by the phenomenon of dosage compensation.
Dosage Compensation is the process by which organisms equalize the expression of genes between members of different biological sexes.
Different sexes are often characterized by different types and numbers of sex chromosomes.The gene expression among the sexes needs to be equalized.
In humans, females (XX) silence the transcription of one X chromosome of each pair, and transcribe all information from the other, expressed X chromosome. Thus, human females have the same number of expressed X-linked genes as do human males (XY), both sexes having essentially one X chromosome per cell, from which to transcribe and express genes.