For many2one fields, 'default_get' only returns the id, whereas
'onchange' (like 'read') returns an array with id and display_name.
Before this rev., when creating a new record, we always called
'name_get' for all many2ones, whether or not their value was
obtained by 'default_get' or 'onchange', i.e. even if their
display_name was already known.
It may just look like unnecessary RPCs, but those RPCs could
actually cause a crash when the user has access to the main model
(and thus can access the display_name of the many2one thanks to
related sudo), but doesn't have access to the many2one comodel.
closesodoo/odoo#30892