Why use temporary web forwarding
If the classic case for permanent web forwarding is moving a website to a new domain, the classic case for temporary web forwarding is during repair or construction of your website. But you might also use it if your website is down for reasons you didn’t expect, like if you’re having trouble renewing your website hosting or you have some technical issue to resolve. You can temporarily forward your traffic someplace else while you fix the problem.
Why use masked web forwarding
In most cases, you should use either permanent web forwarding or temporary web forwarding. If you’re looking for a way for visitors to visit a website hosted somewhere else from your domain name, you’ll probably be better served by either of the previously mentioned types of forwarding or by a CNAME record or ALIAS record in your DNS settings.
There are some cases, though, in which you have a particularly unwieldy URL that you would prefer to hide for the sake of being more user friendly. You can use masked web forwarding to accomplish this. But you should be aware that some browsers will consider this as unsecure and will block it.
Andrew - Domain Names