The difference between using a laptop with the Chromeast vs using HDMI is that with the Chromecast, once the video is started the laptop can be shutdown or you could use it to play a game or whatever since it's not really used anymore (basically becomes a remote to stop/start, pause, etc...). With the HDMI approach, the laptop is tied up for the duration. The Chromecast device doesn't need to be in the same room, just on the same network whereas HDMI is limited by cable length. With the Chromecast, I can use my laptop, netbook, tablet or smartphone to "cast" content.
The Chromecast is actually pretty cool but is held back from a lack of apps to support it.