Let us say you are a small business developing custom web sites and applications to suit the needs of your clients. A tool to create mock-ups will be very valuable to you. Your clients tell you what they want and you create a mock-up and show it to the client and get their feedback. This way when the client sees the finished work there are no surprises. Commercial mock-up tools cost a lot of money. Try the free open source Evolus Pencil. It can also be used to mock-up Mobile as well as Desktop apps.