When you create a new diary group, or visit one,
automatically checks your group parameters and if needed, sets the
summary line format to the diary-specific value, installs the
diary-specific sorting functions, and also adds the different
X-Diary-* headers to the group’s posting-style. It is then easier
to send a diary message, because if you use C-u a or C-u m
on a diary group to prepare a message, these headers will be inserted
automatically (although not filled with proper values yet).
nndiary is a real mail back end. You really send real diary
messages for real. This means for instance that you can give
appointments to anybody (provided they use Gnus and
sending the diary message to them as well.