An Octopus card's a must for use in student canteens at HKU, and photocopiers, vending machines, so on. Not that you can't use cash - diggin' in ya pocket to count the pennies - BUT, at peak times, you'll get frowned upon for holding up the queue/line
If you also register for Octopus 'Reward$', from time to time, you'll get discounts on certain items (as seen in Jimbo's link), plus more points, however 'small', as others hinted at. (Apparently I've accumulated $40 or so, which I can also use immediately as cash towards any future buys in participating shops.)
Local grocery chains (e.g. Kaibo), as well as all the supermarkets, are set up with Octopus machines. (I pick up a few onions, wave my card 'dood' over the pay thingy... it deducts $6, then off I go.)
Handy for paying for 'late' fines at libraries, too.
BTW, convenient if/when car-parking at malls, as well.