But that's weird. Is it safe to be installed or it's some sort of virus?
What it's installing is basically just a shortcut on your desktop (it's more than that, but that's the basic idea). I personally did it on my phone to make it easier for me to check the forum.
In my experience, it is often not the webpage, but the browser doing this. So, for example, my Chrome will say to itself (personified) "Hey, there is an app/shortcut I can make for this page. I will randomly decide whether or not to show a popup about it to let the user know that I can make an app/shortcut for this page!"