Nearly every day there are questions about RAID in this forum. Sometimes I feel, that "having a RAID" is important to nearly every new user.
The first common misconception is, that RAID is giving you security. The second is, that RAID is a backup. Both is not true.
The only important letter in the abbreviation 'RAID' is the first one - and it stands for "redundant". Redundancy is defined by Wikipedia as "the duplication of critical components or functions of a system with the intention of increasing reliability". Your typical RAID is only protecting you against hardware failures of the HDDs in the RAID-array. When one HDD fails, another HDD can keep your system up and running. This could be very useful, if you're running a business: It keeps you working!
RAID is not helping, when you're NAS is infected by maleware, you're deleting accidentally files, you're burglary-victim, your house is hit by lighting or a flood.
You're only investing money in availability. But is availability important to home-users? I don't think so. It doesn't matter if you can view photos now or later. The only important thing for all home-users is, that photos of their children, personal documents and other digital assets will be stored "forever". If you're not sure, what I'm talking about, I would highly recommend to see this TEDx-Talk about the "Lost Generation".
RAID is complex. RAID could cause trouble (see in this section of our forum). Don't waste time and money for availability! Just invest both in a good, easy, reliable backup-strategy. It's cheaper, easier and it is really helpful. And you'll not be part of the "Lost Generation".
With "My thoughts about..." I've started a series on things, I've to write over and over again. These thoughts are total personally. They are not a general advice. They are my personal point of view. Given that, I'll close those threads after writing down my opinion.
@ our other mods: Feel free to add you thoughts to this series. Just use the tag and open you're own thread.