Saving Server Space
We are only allocated 5GB of server space for our site. We are always over this limit. Before I got rid of all the old clutter we were at 40GB
Apart from server space, it is also a good idea to keep your courses as small as possible as this makes them far more portable (should you want to move them to another Moodle site)
This book has a couple of chapters about good practise when creating Moodle courses.