> I literally can think of no reason I personally would want to do this
Being able to swap out RAM and storage and battery from factory-provided can extend the lifetime (and even increase the performance capacity) of a given machine at nothing more than the cost of the new parts. And Apple used to make it pretty easy to do this, and enough people did that there was a subeconomy of vendors selling for this purpose, although of course there were full-service options for people who literally couldn't imagine doing it themselves.
This also meant that downtime wasn't controlled by Apple Store service availability (literally had a drive fail on me once, had my bootable backup, swapped it into the machine, good to go).
And on a number of occasions I've done more complex replacements, a DVD drive here, a keyboard there. I can see why most people wouldn't want to do those, they're a giant pain, but having the option can be empowering.
What's actually hard to think of, if one is thinking, is what Apple has gained in return for this. I can kinda squint and see that irregular (and therefore, perhaps, less efficiently swappable) battery shapes have some credible advantages, but the rest of the marginal ounce-gains and dimensional-golf scores belong to a category of diminishing returns.