Why do molecule clocks use mutations that have no effect on phenotype?

Molecular clocks use mutations that have no effect on phenotype because these neutral mutations accumulate at a relatively constant rate over time, providing a reliable measure of evolutionary time. Since they do not influence the organism's survival or reproduction, they are less likely to be subject to natural selection, allowing for a clearer understanding of genetic divergence. This stability makes them ideal for estimating the timing of evolutionary events and the relationships among species.