1. Cheaper iPhone
2. You don’t want to carry a big iPhone and you have a Mac/iPad nearby anyway for anything complicated so a big phone is unnecessary.
The second case should motivate wanting eg their ‘pro’ cameras in the small phone but the first case motivates making it cheap and low-margin. You can also imagine a world where apple markets a mini phone as also being an optional companion to a bigger phone, but they already have the watch for that.
> 2. You don’t want to carry a big iPhone
If you add those two up, you get "Middle school kids".
Though the apple watch with its own SIM has solved some of the "contact device without instagram" needs that parents want with their kids.