Beacause then you know that the battery is actually good, not some refurbished old battery with patched firmware, or a cheap knock off that’s going to lose half of the capacity in a month, if not outright explode on you.

DO you mean "why get an official battery" or "why do it at a certified store"?

For me I'd do it at the store because it's the only way to make sure you get a good battery. Most cheap batteries are crap (or at least you won't know if it's crap until it's too late).

If it was possible to get a battery that was 100% known to be original, and I could save a few $ by taking it to a corner shop to have the work done, I'd definitely do that for a phone out of warranty. But in my experience this just isn't possible (which is perhaps a better complaint to direct towards apple than the battery hatch complaint: I want to be able to buy original parts in any shop anywhere).

I don’t know if there are any user-visible side effects, but the phone does knows the difference between official and unofficial batteries. Here’s a photo I took of my iPhone 7 showing the internal battery stats immediately after an official replacement during the discount program a couple years ago: https://i.imgur.com/Wo5fSFj.jpg

99.5% of non-OEM batteries for phones and laptops are scams and you can’t tell the difference ahead of time.

Because the phone itself is still fine and works really well, just the battery doesn’t hold a full charge and doesn’t deliver the full output current anymore.

Now I can use the phone for another 1-2 years which is great. I love the form factor as well and right now Apple doesn’t have anything comparable, but I hope they’ll make a small phone again.

Also, the unofficial replacement is not that much cheaper, you get a no-name Chinese battery (which is often worse than the original and more dangerous) and it might break things like FaceID/TouchID because the security chip will be reset and the non-official repair shops don’t have any means to reprogram it (as far as I know).

Labor is expensive. The cost difference between OEM and aftermarket parts is small.

If the aftermarket battery is fine, you only gain a little. If the aftermarket battery disappoints, you lose big. So it's cheap insurance.

Does the replacement work nave a warranty?