To oversimplify...

In studies you are looking for what is known as statistical significance, which is basically shorthand for it being very likely that the effect you are seeing is a real effect rather than just being an unlikely chance effect.

If you are looking at the effects of a drug, perhaps the effect that you are seeing is just random chance - the people who took the drug just randomly got lucky and the people who didn't take the drug got unlucky.

So you do replication to rule out that chance. If you do two independent drug trials and they both show the same effect, the chance that it is due to random fluctuations is much smaller.