A common approach on Android. Nobody really seems to know exactly why, but evidently crap accumulates, either from old apps or from sloppy in-place OS upgrades. A full reset clears all the crap, and you start anew.

I prefer to skip even the backup step, instead relying on cloud sync for contacts etc, and manually dumping a couple apps.

Yes -- give it a try. My iPhone 7 Plus was nearly out of storage space, and also rather sluggish. Much of the storage space was being consumed by internal Apple files according to usage details within the Settings app.

After doing a full backup using iTunes, then resetting the phone to factory conditions, and finally restoring the iTunes backup -- my storage space returned to normal, and the speed increased dramatically.

I suspect that over time, internal files accumulate (e.g. log files & upgrade restoration points), and this accumulated cruft gradually consumes space & degrades performance.

Yeah it made it soooo much quicker. I was about to throw it in the bin. Now set for another year of retro phone. Mine was being really shitty though, it took a massive speed nose dive after ios11. If that’s what happened to you defo worth trying.