@Crell honestly mostly trying to avoid having two copies of the call to foo() in an if/else with different argument lists. (way more obvious why in the real case with more params to foo.)
It really boils down to "this library allows you to specify an old format version and defaults to it's current one, which is an internal detail." vs "this function lets you specify the version of the object you're dealing with, in case you need to use an old object from storage"