Every new model year brings better cameras, faster and smoother overall operation and full support for all the new functions of the OS.
Most iPhone users I know hang onto their phone until the battery needs replacement or the phone they have falls out of support for the new OS. That's usually in the 5 to 6 years range.
By contrast, the Android users I know tend to replace their phones every 2 years at most, due to performance degradation, support for their phone ending, leaving them no choice if the depend on applications that move on from supporting the OS version their phone supports, and just general shitty hardware issues. Companies like Samsung putting out a dozen models a year means that they don't bother making sure what they put out works well or for very long, as they need you to buy a new phone to remain profitable, since Google doesn't cut the phone makers in on the money from the Play Store.
In corporate environments, it is not uncommon for users to skip upgrades if they have iPhones, but the Android users are always needing approval for replacements well before their entitlement dates, always for valid reasons relating to loss of function or diminished performance. One non-profit I worked at actually dropped Android for employees completely due to the cost of the replacements going insane.