Since you asked:
The NAS I stated will use about 30 watts and will cost about $31 a year to run. Add two more years to the calculations if you want. But after the seventh year it is $31 a year vs. $120 a year. I will add these NAS do a lot more than hold your files and data. They run a lot of software and can host a Plex server, photo sharing features, collaboration apps. The more powerful ones can work as virtual machines. Host websites, etc. Also, I now buy computers with less storage space on them becuase I know I'll offload my files to the NAS. All our home Macs have 500GB drives. Really not very much storage but we only keep files we're using on the local drive. When a project or task is done the files get moved to the NAS for archiving. This saves a lot of money on drive space which Apple charges a steep premium for. You start adding all this up and the NAS becomes a no-brainer, in my calulation.
To back up your iPhone and iPad, you need to use a Mac or PC. Then you back those up to the NAS. It is easy to use time machine on the NAS. All our family has Mac's and we each back up on time machine to the NAS. You could buy separate external drives for this task. But if that's what you've been doing, you can eliminate the expense of all those individual drives. The process is similar with a PC.
I mentioned running in RAID 5 for redundancy. I've been using mine for over six years. Never a failure, unless you include power outages. At the same time, Apple Services are not 100% guaranteed either and go down from time to time. Enterprise level drives have a very low failure rate. I also own a second NAS that is at my brother's house and my home NAS backs up everynight to the other NAS. This level of redundancy helps if my house burns down or my main NAS gets hacked!
When I'm about to replace a phone, I do two backups to iCloud before I go in and restore from the cloud at point of purchase. But more people today get their phones delivered to the home and you could avoid that step.
I did not know a lot about what a NAS can do when I got my first one. My main motivator was to consolodate all my files and photos. I have old computers going back a few decades? What a pain to find all my old files and memories. With a NAS, I save everything to that and it will be there for the rest of my life. But once I got the NAS and started playing around with it, I learned they can do so much more. I like Synology and you can learn more about all they can do here:
https://www.synology.com/en-us/dsm