This is a little bit of a rant, so be prepared.
I have lost count of the number of times I come across people in the “biz” who tell clients, or anyone else who will listen, that replication is the same as backup. It most emphatically is not! To tell somebody that is to tell a lie, pure an simple.
Backup and replication can be equal parts of an overall DR strategy but they are very different animals and do very different things for very different reasons.
A backup is a “snapshot in time” of the state of files and/or a system. It is a static picture of how things are at the point in time that the backup is taken. Once the backup is taken it should be immutable, unchanging. It is as things “were”, not how they “are”.
Replication, on the other hand, is fluid. By definition, if I am replicating from one system to another then I am copying the continually changing state of system “A” to system “B”. If I add a file to “A” it should show up in the same place on “B’”, possibly in near realtime. If I delete the same file on “A” then it will be removed from “B”. I cannot rely on “B” having a copy of a file that has been deleted from “A” nor can I rely on “B” having an unchanged copy of a file that has been modified on “A”. As can be surmised, replication is as things “are”, not as they “were”.
Backups save your butt when things screw up and files and/or systems get trashed. Backups help all of us save face in the case of the file delete “oopsie”. Backups let us get back to a known point in time, if needs be. And, yes, backups can allow us to recover full systems albeit usually with moderate to significant time lag. Backup is all about “recovery”.
Replication can ensure that data is always available because more than one current copy exists and can be made accessible. It is about having the ability to “fail over” to another system because everything that is needed is available in more than one place. Replication is all about continuity and availability.
The definitions I have given here may be a bit simplistic but you should get the drift. And remember, replication ≠ backup; backup ≠ replication.