gpg-agent(1) says:

You should always add the following lines to your .bashrc  or  whatever
initialization file is used for all shell invocations:

  GPG_TTY=$(tty)
  export GPG_TTY

It is important that this environment variable always reflects the out‐
put of the tty command.

As i wrote over on https://bugs.debian.org/847389,

Always setting this in .bashrc implies that the current tty is published in the environment.

what happens with this configuration if (for example) the user:

• launches a terminal emulator (system allocates /dev/pts/14)
• in that terminal, runs thunderbird &

at some point in the future, thunderbird is going to talk to enigmail, which is going to invoke gpg, which might or might not talk to a running gpg-agent.

Ideally, thunderbird doesn't try to tell the agent to talk to the terminal to prompt the user for their passphrase.

even worse, if the user:

• exits that terminal

and then later another pty is allocated on that machine which happens to grab /dev/pts/14 (e.g. imagine some other user ssh's in from the outside in an interactive session), then we definitely don't want gpg-agent to try to talk to that user's pty. (normally, they wouldn't have the permissions to do so, but that user could chmod o+rw /dev/pts/14 to scoop up the communications and use it to test passwords against the agent). This isn't a terrible breakage, but it doesn't seem like a great idea either.

furthermore, having pinentry prompt the user on the TTY is known to not work well if the tty is in use by another process. See the upstream discussion.