[odammit]

Off the top of my head: load balancing, hiding details of app servers, compressing responses and multivariate testing.

All of which could be done at the app server level sure, but then that would shift that complexity to your app and your developers.

Oh and job security, obviously.

[fulafel]

You could do all of those, except hiding app servers, with the client based technique I outlined in the nearby other comment. It would just be a tweak to the rule that the frontend uses to choose the the app server.