Tiling saves a lot of time if your workflow tends to involve multiple windows.
Everything is edited through one config file, so things are a lot easier to keep track of and integrate, whereas in openbox you have a couple config files for openbox alone, plus ones for whatever trays/panels/etc. that you are using.
The more I learned to not use the mouse, the faster I got things done with awesomewm. (much faster than I ever did with openbox).
tags > workspaces (you can combine/uncombine different tags at will with awesomewm, with everything on each selected tag nicely tiling for you, for example)
I suppose that initially it is harder to configure than openbox, as there is no equivalent of things like obmenu and obkey for awesomewm (though im sure not everyone uses those).
Some people think tiling wm's are ugly and like to have window borders. I strongly disagree, but... some people say that.