The Reader will allow you to:
Use a USB mass storage device without FireFly's Drivers for GigaDrive
Improve your read and write speeds
Not have to plug your GP in when loading SMC
The USB cable will allow you to:
Use GigaDrive, which is just as good as a card reader if you have XP or the drivers
Use the Mouse and ControlPad Drivers
Unpack FPK's without the unzip thingy
Rumour: Play future versions of KOF91 over the net.
EDIT: And of course, if you ever need to, sell your GP32 as a full set.