The main case where this helps is byte strings. GVariant has a special
case for values of type 'ay' which end with a NUL byte and otherwise
contain only printable ASCII characters: it prints them in the form
b'asciistring'. UDisks and Flatpak (among others) use bytestrings of
this form extensively for file paths, which are not guaranteed to be
UTF-8 so cannot be represented as D-Bus type 's'.

This does have the downside that b'foo' is also valid Python but means
something different:

In [2]: GLib.Variant('ay', b'abc')
Out[2]: GLib.Variant('ay', [0x61, 0x62, 0x63])

In [3]: GLib.Variant('ay', b'abc\0')
Out[3]: GLib.Variant('ay', b'abc')