There are several reasons for doing that:

1. It documents the code for the reader and helps find
   inconsistencies and bugs.

2. For rej_perms, it guarantees the change will be done
   even if the output reference can be created by several
   code paths.

3. It can be used to predict cases where a copy will,
   or will not happen and optimize buffer allocation
   (for example not request a rare direct-rendering buffer
   from a device sink if it will be copied anyway).

Note that a filter is still allowed to manage the permissions
on its own without using these fields.