It allows to literally “embed” the website in another website.
If you have ever seen a Youtube video in a forum or on a news page, although the forum or news page did not seem to be owned by Google (conspiracy!!!), that was because a small rectangular region on the page “embedded” the youtube page showing the video.
Similarly, wherever it is possible to “embed” such content, you can now also showcase your SketchUp model. For example on your website! The reason why there are different URLs is that the embed page only shows content (no superfluous white space and menu and scrolling) and probably has some safeguards so that the host page cannot access the 3DWH user’s personal information.
Usually the embed URL refers to a specific model (not the main page), e.g.:
Ok, thanks for the explanation!