Why is it necessary to sign two applications with the same digital signature?

So that two applications can have the same sharedUserId. This allows them to run in the same process and share data directly, bypassing Binder.

The second scenario is updating an already installed application. The new version must be signed with the same key as the previous one so that the system considers it a legitimate update.

The third is the ability to access each other's privileged APIs and data if they declare the appropriate permissions with the attribute android:sharedProtectionLevel="signature".

The fourth is integration and sharing of components within a group of applications developed by the same manufacturer.