I was going through some of my graphics folders and came across some really great designs that I wanted to use for other things. These were projects I started when I first started learning graphics that came out really good. In order to save hard drive space once a project is completed I would save it as a jpg file, or gif file instead of say an xcf file for Gimp or a psd file for Photoshop.
Yup, it does save some hard drive space. The only problem is when you want to use those designs for other projects they pretty much become useless. I would strongly suggest to anyone just starting out, save your work in a project file. Use xcf, psd, or whatever your program saves them as so you can edit them in the future.
Sometimes you may want just a piece of clipart from the project. Maybe a background came out just right and would make a nice tile. You get the point. Saving your work in a project file makes all these options available to you. Note to self and anyone else just learning: Save project files in editable format !