Why is replication in scientific experiments important?

If you can perform the exact same experiment multiple times, and get the exact same results every single time, it is much easier to draw a confident conclusion from your results than if you only sometimes got the same results from performing the same experiment. Since many experiments test the effect of one thing on another, having consistent results demonstrates that a change in one variable consistently affects the outcome of the other and add validity to whatever rational conclusions are drawn from the results.