Along with some of the other good reasons up there, I belive the main reason of confirming positive rate of climb is to ensure that you are in fact climbing, there are many reasons may have you thinking you are climbing, for example, surrounding terrian might give you false indications, a big one that most people forget is at night you might even feel you are climbing , yet you may even be decending.
Another point, what if you are incorrectly loaded, you pull back on the stick, assume that you flying, yet you are still trundling down the runway.
There are more reasons, but hey how many do you need to realise that it is an important call.