Throughout your time at university, you will have numerous different projects. When you partake in these projects, you typically do them in the scope of the module that assigned them, using techniques and knowledge that you have learnt from that module. As you progress, you will learn more and more. Some of what you learn can be applied to prior projects.
Going back and revisiting your prior projects to improve them, can offer several benefits. Having learnt new skills and consolidated prior skills and knowledge, you will be better prepared to take on that project. Through applying new skills and techniques, you can build a project that is more robust and that does more. For example, you may have worked on a project to build a relatively basic employee management system, you may want to add a GUI, a database and database handler, and new features.
In doing so, you can also build a project that would be good to have on your portfolio. By taking an older, more basic project, and building it up by adding different technologies and tools to make a comprehensive, finished product, it would be a good project to display on your portfolio.