1. Application upgrade: If you want users to upgrade to the new version seamlessly, you must sign with the same certificate. This is because if the upgraded application is signed with the same certificate, only the upgraded application will be allowed to be installed. If you use a different certificate, your application will be required to use a different package name, which is equivalent to installing a brand new application. If you want to upgrade the application, the signing certificate should be the same and the package name should be the same!

2. Modularization of applications: Android system can allow multiple applications signed by the same certificate to run in one process, and the system actually treats them as a single application. At this point, our application can be deployed in a modular way, and users can upgrade one of the modules independently.

3. Enjoy code or data * * *: Android provides a signature-based permission mechanism, so one application can disclose its functions for another application signed with the same certificate. By signing multiple applications with the same certificate, code and data can be safely shared between applications by using signature-based permission check.