Hmmm... I think my XBONE controller did update many times, not sure if the DS4 also did, perhaps.
I don't think the have to update, but they can.
Like almost every product nowadays, people sell works in progress.
Be games, that later are "improved" with zero day patches, or hardware pieces that also are "improved" by firmware updates.
A firmware update could improve battery life, improve wireless quality, etc. (but sure they won't fix drifting sticks whose internals star to fall apart/s)
If you mean make it work better / use less battery, they sure can:
e.g. Suppose they configured the accelerometer in a higher sample rate, but then they realize it is enough with a lower sample rate that uses less battery and then they create a firmware update with this behavior.
If you mean "stability" as in annoy hackers, perhaps there are many ways, but imagine an additional annoyance/deterrent to e.g. downgrade a firmware, as in make the joycon use a protocol that an older firmware does not understand, or make it use a new protocol that unofficial joycons do not use, so the unofficial ones stop working on new firmwares, etc.