Here's my experience with the "magic box".
Car is a 2008 Mercedes CLK 350 convertible. Air conditioning isn't working. Owner (me) unhappy about this. Climate control on this car is seriously complicated, lots of options, temp individually adjustable on each side, blower also, vents also, this is kinda nuts. Standard Teutonic over-engineering.
Plug in magic box, set to correct vehicle (standard plug, software internally selectable for about 50 different models).
Hit "go". Box scrolls down and does about a hundred (!) different scans and checks, stops at "climate control".
"Enter for further information", OK, tell me.
"Module 1131 out of range, enter for further information"
Click.
"Humidity 0%, outside air temperature -40F"
In Florida. In the summer. Right. I doubt it.
A few minutes with Google yields the correct MB part number for module 1131 and where it lives (front of the firewall, next to the battery).
List price on the module is over $400. eBay has a guaranteed used one for - ready - $12 delivered! Buy it now. Four days later it is here, three screws and one plug and it is in, A/C now works flawlessly.
The dealer would have charged me an absolute minimum of $150 in diagnostics, plus the module, plus tax, and I'd have had to leave the car with them for a couple of days. If I had been able to get out of there for $750, I would have been happy. My net cost was $12, and I don't count the $50 cost for the box because I can use it again. (They would have washed the car as part of the $750, but man, that's an EXPENSIVE car wash!)
So I know it works.
Minimal learning curve, no special tools needed, quick repair, absurdly low cost - how the heck do you fix ANYTHING on a Mercedes for $12? But I did . . .
While it is entirely correct to say that some repairs are going to cost a lot more and take a bunch of special tools and training, there are also a lot of simple (and some not-so-simple) things that can be done by owners, who can save a LOT of time and money doing it.
What the "magic box" does is talk to the ECU in the vehicle and read stored trouble codes. We can't look at a sensor or a control box or a catalytic converter and tell if it is good or bad, but the computer knows and the box will tell us. OBD II and its various relatives save a huge amount of diagnostic time and keeps mechanics from just throwing (your/our) money at some obscure problem until it is solved.
Lets extend the function of the box to check dealer stock on the part - that isn't much of a leap from what we have now. You've already told the box what model you have, so there's no reason there can't be an electronic parts lookup function. Your phone has a GPS in it, so it can use the Google "Vendors near me" function to find the nearest dealer or dealers.
As to needing connectivity, it could ride on your cell phone connection, so no computer or separate satellite equipment needed.
Older and simpler machines don't (usually) have ECUs, but if your (whatever) does, you'll be very thankful for the code readers.
Best Regards,
Mike/Florida