What is the purpose of the /etc/apt/preferences.d directory in Ubuntu?

When I put some apt config file into /etc/apt/preferences.d of my Ubuntu 20 LTS desktop, it is ignored (cannot be seen in the output of apt-config dump).
When I instead put the same *.conf file into the /etc/apt/apt.conf.d directory, it is honored (can be seen in the output of apt-config dump).

So what is the purpose of the /etc/apt/preferences.d directory in Ubuntu ?

According to man apt_preferences, the directory /etc/apt/preferences.d is for *.pref files that contain preferences for packages, but preferences for apt-config must go into apt.conf.d/*.conf.

apt-config ignores wrong *.conf files in /etc/apt/preferences.d, but apt-get install warns with
Ignoring file 'somefile.conf' in directory '/etc/apt/preferences.d/' as it has an invalid filename extension.