What is the aspnet_client Directory Used For?

The aspnet_client directory holds important javascript files used by ASP.Net sites for client side validation on web form controls. Although the directory isn't required for an ASP.Net site to run, it will affect behaviour of certain controls with client side validation enabled, so removing the directory should be avoided (unless you know what you are doing).

If you've removed it by mistake you can regenerate it by reinstalling the ASP.Net site using:

aspnet_regiis -c