Checkout how Download Source Code feature on Extensions in Business Central Version 15 Production helps customers.

Problem:
A friend of mine was looking to migrate their Business Central development and support from one partner to another.
While, asking me for advice, I was evaluating what was the best solution knowing that their Business Central had customization installed and I found this which made it easy for them as their going to continue customizing Business Central from another partner.

Solution:
In Business Central version 14 Sandbox, you are allowed to Download Source Code.
But in Business Central version 14 Production, this was not the case. No Apps. could have been downloaded even though you got it developed from a Partner/ Developer and installed from them.In Business Central version 15 Sandbox, you are allowed to Download Source Code just like you would in Business Central Version 14 Sandbox.

Download Source Code for Installed Extensions in Business Central V15 Sandbox

And this is also the same condition  for Business Central version 15 Production tenant as well.

Download Source Code for Installed Extensions in Business Central V15 Production

Without this, the possible solution to get your app source code back from Production, you would be required to make a copy of Production into Sandbox and download it from there in Business Central Version 14.

Note: This doesn’t apply to apps installed through MarketPlace.  Trust me I have tried ?!!!

Conclusion:
When App was installed in Business Central Version 14 Production ensured Partners / Developers that app couldn’t be reused on other tenants.
This put most of the pressure on the Customers to explicitly to ask the Vendor to provide the App File.
Whereas when an App is installed on Business Central Version 15 Production, it can be downloaded and installed in another Business Central Tenants.
This means that if the Customers want to change their partners, they can seamlessly do this without having the awkward conversation of asking the code from their previous partner.