Science isn't a perfect process; it is done by people, and people make mistakes. Things go wrong, people miss things, or they just get really unlucky.

Replication helps control for that.

You get a different set of people to do the same experiment in the same way, and you should get the same result. If you do, that's a good sign. If not, that's a problem and something that should be looked into.

Ideally then you get a different set of people do the same experiment in a slightly different way. And then a different experiment that measures the same thing, and so on. Lots of replication, all aimed at controlling for things people didn't think about or didn't spot.

Now you could get just one team to do this, over a long period of time, and include it all in a single study, but that is kind of inefficient. Better to do each study separately, then you can publish them individually and other people can have a chance to look at it as well. Plus it is generally a good idea to get a second person or team to work on something.

In a perfect science world you never stop experimenting on something. You never treat it as fully settled, you keep testing your idea until you disprove it, keep trying to find new ways to poke at it and experiment on it.