There is a subtle difference. git-show is meant to show an object. A commit is an object, but so are other things. A more descriptive name would be 'git show-object.'
An example that I just came across in normal usage:
git show <branch_name>:<filename>
This will show you the contents of <filename> on <branch_name>. It does not show you the diff of that file for the latest commit to <branch_name>; it spits out the state of that file to the $PAGER. This is something that 'git diff' does not do. Note that <filename> doesn't even have to exist on the current branch, and you don't even have to have a working tree (i.e. you can do this in a bare repo to inspect file contents without actually needing to checkout a working copy of all of the code).
