If your data is important to you, you need to have a solid backup strategy to protect that data against failure or natural disasters. There are two kinds of backups and both are important.
The first type of backup you need is real-time protection against data loss. This is data redundancy and can be accomplished with either a RAID system or mirror drives. With the cost of hard drives so low, I recommend against any kind of hardware or software based RAID system. The reason is compatibility for the future. If you implement a hardware based RAID, it may not be recoverable if the hardware fails. With the fast pace if technology, you are unlikely to find an exact replacement when it does fail. Software based RAID is also problematic if you upgrade your operating system. Mirroring used to be expensive but not any more when hard drives are under $100 for 2 terabytes. Just slap on another hard drive and mirror your primary data. You’ll have real-time protection against a hard drive failure.
But having a mirrored drive does not protect you against user errors and natural disasters. If you delete all your files from last year, the corresponding files in the mirrored drive will be deleted as well. It will not protect you if your home burns down. To protect against such scenarios, you need two off site backups.