When you are asked to do this, its so the computer can read what you have on your memory card when plugged into the usb port. In this mode, you can transfer (drag & drop) media without going through desktop and media managers. It turns your phone into an extra storage drive so to speak.
The prompt only comes on when I'm connected to the pc. I think I will make it default answer "yes" so I won't have to answer the prompt everytime I connect to the pc. Thanks!
If you don't want your phone to ask every time like me all you have to do is go into your settings then select media card and select yes to allow to be a multi media card