You can share just one of the windows you have open or everything that you see on your computer screen while having a conversation on Skype. Why would you need it? It is convenient for doing homework or a project with someone who is not sitting next to you.You can show them what you have come up with or have them watch as you are doing it. It is so much easier to show someone your desktop if you have a computer problem rather than trying to explain what it is. Grandma or a friend is having trouble using a program? Show them how. It is not difficult to think of many reasons to share.