What is a user's primary group, and why is every user also a member of a group with their own name?

The primary group is the GID that new files get; Debian creates a private group with the same name for each user (user private groups), so their files are not shared by default.

On Unix, access is meant to be given to groups: a user belongs to one primary group (the GID in /etc/passwd) and any number of supplementary groups (/etc/group). The id command shows all of them.

With user private groups, adduser bob also creates a group bob that has only bob in it and makes it his primary group. The benefit: bob's new files are owned by bob:bob, so even a permissive umask that gives the group write access does not open the file to anyone else. Sharing happens on purpose, by putting people into a shared group (for example finance) and giving files or directories that group, often with the SGID bit on the directory.

Go deeper:

• Wikipedia: Group identifier — primary vs supplementary groups and user private groups.