Some advantages to have some Web Player :

- Share quickly any very early gameplay or level demo without needing to download or install something :
Just click on the link

- Use it to share and show your 3D art models with lightening letting the user use camera to check all details !
(usefull if you work with a team also )

- For people newbbie to IT , just a link to click on (once you have the plugin on)

- For a demo you don't need to have all the work to make the install package ect ... just publish a Web version, for
people can to play and give you feedback

Well, i just sayed it's usefull for showing your models, a demo to people !
For a real big game, installation is better indeed !
You don't have to download 1Go each time to play !