First of all,
sudo vim already explicitly mentions your default editor, which is not necessary if you have it defined in
sudoedit spares you from defining the editor every time you want to edit something—and on a multiuser system it allows everyone to use the editor they personally like.
How so? Consider a system where normal users only get
sudo privileges for editing certain files. They are restricted from running
sudo with anything else though. You would have to allow them to
sudo vi and
sudo vim and
sudo nano and
sudo emacs and
sudo pico (et cetera). Instead of having to do that, you could simply allow them to
sudoedit the file, with their choice of setting
$EDITOR to whatever they like. (Imagine you'd force an Emacs lover to use Vim…)
Another issue is that if your
$EDITOR is set to
vim, and you have customization settings for it in your user's
.vimrc, those settings will not be used if you use
sudo vim or
sudoedit however preserves the calling user's environment, and therefore your settings.
See also: What's so great about sudoedit?