Because when you're doing anything outside of git commit am- "whatever" it's nice to be able to see the complete implications of your actions, what files have been modified, what lines in those files have changed, etc.,
git commit -v gives you the diffs provided you don't also use -m. Personally, I find -m to be a dangerous flag for commit for exactly the reasons you described not mention its hard to really do detailed multi-line commit messages with -m.