When are Avalanche-Rated MOSFETs Necessary?
First of all please note that there is no industry standard for doing avalanche rating, and that often avalanche rated FETs are tested under rather unrealistic conditions (e.g. single pulse only whereas using them in repetitive will cause self destruction). However characterizing and hardening it is sometimes not much harder, in fact there are some transistors that are not rated at all but known for having usable avalanche behaviour.

For the classic scenario of driving something inductive, I can imagine multiple reasons why you use such a transistor:


A bit of extra safety for when the flywheel diode fails.
Your chosen transistor has it anyways already and you can shave off costs by omitting the diode
You are so space constrained that you can't put a diode there.
You have other sources of voltage spikes that you have little to no control over (e.g. surrounding engine ignition systems) but want to be able to cope with.


As such it doesn't often seem to be a base of a good design, but there is a market for it, and so people cater this market.