um ...
i think it lets the email continue to be handled by the original ISP, just forwarding the http to the other place. ... but i may be wrong! and even if i'm not, i know you can do it with the mx thingy.
i guess it comes down to lazyness ....
in my case, it saves me setting up DNS servers. (on reflection ... this is the real reason!)
It would also be one thing to change instead of 2.