the mini display port connection on the side of a mbpro is also an input -output for the new thunderbolt tec,ie it does both,thunderbolt has an input output rated at 10 gbp/s[theoretical] and can be daisy chained with up to 6 devices,including multiple monitors, so its another way for you to connect a hdrive [external]monitor,or other peripheral with said connection,
at the moment products are only just emerging with this new form of connection,however,as in the future more will be,since it far exceeds usb 1,2,or 3 i/o speeds,
it is on the new issue of mbpros so far,probably will come on new issue of imacs too when they come shortly,
its like usb but kills it for speed,and has a different adapter[ie mini display port]on macs with it,
it will means backups could takes minutes instead of hours,
so far as i know anyway,i havent used it yet,cos not a lot of stuff is developed for it bar a few external drives ive heard of.but they will come.