Why does `cabal configure` bother to create a `cabal.project.local` that only contains `ignore-project: False`?

What's the point of a `cabal.project.local` that only contains `ignore-project: False`? Why does `cabal configure` bother to write it (and possibly destroying the user configuration there, #8408)?

What is the point of `ignore-project: False` in a project configuration file anyway?
(It is like sealing the message "have this letter opened" into an envelope. Either you already have opened the letter, or you haven't opened the letter in which case you do not see the message. :-))

Start at the `cabal configure` entry point and trace why it writes `cabal.project.local` with only `ignore-project: False`. Investigate the interaction with existing user configuration, including the concern in #8408. Done means documenting the intended behavior and defining whether configuration files should be preserved or omitted.

Written by the indexing model from the issue text.