I see a bunch of explanations, but I don't think any of them really drove home the reason for why you usually need both.

Imagine if you had a document that stored useful information. You had this document automatically replicated to another system in a different time zone every time there was a change.

You think you're doing great, if there is an outage in the one system you just get your important document from the other system.

Then one day someone accidentally copies and pastes the wrong data into the file. Now what do you do? If you goto your replicated copy it also has the bad data.

The answer is you should have had backups too. so you could go back an hour,a day or a week or even much longer. Depending on how much data you have, how often it changes, how important it is, and how quickly someone would notice bad data.