It's not used for anything anymore, and may well end up being removed.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/808
// Miscellaneous monkeypatching
_patchContainerClass(St.BoxLayout);
- _patchLayoutClass(Clutter.TableLayout, { row_spacing: 'spacing-rows',
- column_spacing: 'spacing-columns' });
_patchLayoutClass(Clutter.GridLayout, { row_spacing: 'spacing-rows',
column_spacing: 'spacing-columns' });
_patchLayoutClass(Clutter.BoxLayout, { spacing: 'spacing' });