Why is Instance.Parent marked as notreplicated?

In the manual the Instance.Parent property is marked as notreplicated and says that this item is not replicated on the server/client boundary, but if you change this property on the server the change is replicated to the client. Could someone explain to me what it really means?

It means if the change is made on the client, it wont replicate to the server. It applies if it does not replicate both ways.

After reading that publication I understood that properties marked as notreplicated are not really replicated, but rather calculated internally. The result is apparently the same as if it had been replicated. So, from a scripter’s point of view, what is the point of marking properties as notreplicated?

Yup, the properties marked as notreplicated are not really replicated.
If by a LocalScript you change something, that change will not be visible for the server, so the other clients will never see that change.
The point of marking “not replicated” is for us to know if that change can be replicated to server or not, or viceversa, in order for us to decide the approach on each system we code. Without that info, you could decide an entire system that “supposedly works” and when you test it with real players, you will notice an epic fail on your system.

Deciding what to change on what side Server or Client is very important, and knowing what replicates or not its useful information