When you have written a book or a song, taken a picture or created a design, you may want to post it on the web immediately and share it with others. However, it only needs one unscrupulous person to copy your work and claim to have created it themselves.
If that happens, you could find it difficult to prove that the work is, in fact, yours. Don't let this happen to you! Register your work before you air it publicly. Even if the work is not yet completely finished, it's good to register it at different stages of its evolution.