'User agent' is a great idea that has been weirdly perverted.
Nobody these days (even highly technical people) has a user agent. (Maybe @drwho does.)
A user agent is a piece of software controlled by the *user*, that performs the automatic tasks the *user* has instructed it to. It communicates with other user agents, automatically, on the user's behalf.
Today, the term 'user agent' means 'long, misleading browser-lineage-identification string'. It identifies one of ~3 corporations.