"Victoria Dye via GitGitGadget" [off-list ref] writes:

About a year ago, discussion on the sparse index integration of 'git grep'
surfaced larger incompatibilities between sparse-checkout and submodules
[1]. This series fixes one of the underlying issues to that incompatibility,
which is that the worktree config of the submodule (where
'core.sparseCheckout', 'core.sparseCheckoutCone', and 'index.sparse' are
set) is not used when operating on the submodule from its super project
(e.g., in a command with '--recurse-submodules').

OK.  So in short, worktreeConfig used to be a singleton, global for
the entire git process, but because "git grep" (and possibly others)
wants to operate across repository boundary when recursing into
submodules, the repository_format_worktree_config needs to be per
repository instance, not a global singleton.

Makes sense.