The SE has similar internals to the 6s, but is the size of the iPhone 5s. They’re extremely popular with people who love the size of the 5s but want a faster processor and better camera than the 5s had.

Main difference: smaller form factor, flat sides. This alone makes it easier to hold for people with small hands or motor coordination issues (eg: elderly people).

Older TouchID Gen1 sensor mainly. Same SoC, was advertised at the time as an option for those that prefer smaller sized phones.

Easier to use with one hand.

And barometer, strangely, as well as Touch ID sensor.