The MOST likely reason the developer might want to define their own permission in the manifest is
because:

A.
they wish to ensure that only their app has the permission to launch their activities or access
their private data.

B.
they wish to prevent the user from granting access to protected functionality by mistake.

C.
they wish to define a permission to access system APIs and native libraries.

D.
they wish to restrict access to a function in their app to only those apps which are specifically
granted access by the user.

Explanation: