How about someone found out your IMEI and you think they're going to track you and steal the phone or worse? How about you just don't want tracked all the time by Google etc? My phone, I can do anything I like to it.
If one is really serious about not been tracked, maybe use burner phones, and stay away from Google, Facebook etc.
All the apps I looked at seemed to have people claiming it worked. Or just the instructions to do it in the engineering mode without an app.
I guess I'd have to flash to change mine then.
I doubt very much if it could be flashed either. Because the IMEI, MEID, etc. is stored in OTP(
one-time programmable) ROM in the chip-set. One-time, meaning once it's written to, it's permanent, and can't be written to again. Really the only way to change that, would be to physically remove and replace a chip on the device's logic-board.
If you want the technical details, look at some of the relevant patents for this tech, e.g.
https://patents.google.com/patent/EP3664490A1/en
"The invention provides a chipset for a terminal (ME), the chipset comprising at least one secure processor (SP), into which a one-time programmable memory (OTP area) is integrated, with at least one terminal serial number (IMEI) of the terminal (ME ) is stored, with information for securing the terminal serial number (IMEI) against manipulation being stored in the one-time programmable memory (OTP area)."
BTW in the mid '90s I had an OKI ETACS analog phone. To change the ESN on that involved de-soldering the OTP ROM chip from the board. Obtaining a new blank OTP ROM chip, programming it with the desired ESN using a programmer and computer, and then soldering it in the board. So the business of making it difficult(preferably impossible) to change or clone an ESN, IMEI, MEID is not a recent thing.