You want to turn it back on to validate everything is working right and it’s all plugged in correct. You also want to validate you didn’t break anything else while attempting the repair
And also(sadly necessary) you want to stop scammers who will send you a broken phone and then pretend you broke it. So you want to switch it on before and after the repair.
Especially the screen. I mean you have to rip off the screen to get to the internals. Lots of people already have cracked screens and it is hard to put them back on without making the cracks even bigger. A small crack with no chromatic effects can easily make those from the prying. Why would you not want to validate the quality of the screen after you have just been wrestling with it?
Oh man, HTC 10 was the worst phone I owned in terms of repair-ability (I even saw one phone repair place explicitly saying they don't support that phone). Not only you risk screen (primarily) and other components damage. You can also break the phone in subtle ways when putting it back[1] and reopening it back is again tough because you need to unglue the screen first to get to screws.
Having a builtin validating code as one commenter mentioned would be a godsend, but nearly all companies do everything they can to make customers not want their phones repaired.
[1] things like some sensors not working, accidentally clipping the tape with buttons, touch screen being funky (although that likely was due to non genuine screen), or my favorite - gps working but never able to get exact location)
I destroyed an iPhone 6 Plus screen one time reassembling it. Apparently I switched a 1.7 mm screw with a 1.3 mm, and when I popped it back together then entire screen (which wasn’t working for touch anyway) shattered.
Can't they have a signed "self-test" image that they can boot up and it checks the phone? This wouldn't have access to any user data but can boot up and check that all of the components are working. This sounds much quicker and more effective than prodding at the user's OS anyways.
At least on my phone (Samsung), the "self-test" app is on the /system partition, which is where the user OS is, and you need to boot normally to use it. You can run it by entering *#0*# in the dialer, no need to flash anything.
Having a dedicated test image (like /recovery) is a possibility, but it wouldn't be the same environment as the user. The kernel may be different, maybe some runtime calibration data would be missing, and most customers want to see their phone working after a repair.
In high volume test and repair environments this exists, but part of how they can do it is they erase and overwrite the data on the phone. When you don’t care about the users data this becomes much easier. Data migration is a pain. (Esp when it’s not yours)
In my case they ran a diagnosis over the network to make sure everything is working.
Diagnosis software is built into iPhone so I can put a trust on it that it ain't sharing private data to the store employees.
Similarly they ran a diagnosis again at the end of repair. They did boot up the phone my themselves and ran it. Looks like they can run diagnosis on locked Phone.
This is overall much better than asking to unlock Phone.