MRNA degradation is one of the most important mechanisms living cells use to regulate the expression of genes by the machinery of the cell. By selectively degrading messenger RNA, cells prevent the inappropriate, and potentially disastrous, expression of certain genes.
Most cells carry a full complement of genetic material. This genome is "read" in full by messenger RNA. In multicellular organisms, full expression of every gene in every cell would cause catastrophic malformation and impair tissue function to the point of fatality. By breaking down the mRNA that "reads" the code for unnecessary genes, the cellular degradation machinery permits each cell to express only the genes that are appropriate for its role in the body.