It's not really possible to reason about this with the information given. DDoS attacks can range from a few megabit per second to many gigabits -- it all comes down to how sophisticated and resourceful an attacker you want to protect against.
If our main ISP Backbone provider have no anti-DDoS mechanism, means that there is no point we get the Arbor Pravail?
Mnn, not quite. If your hosting center has decent uplinks, and you're deploying an anti-DDoS appliance, then there is a good chance that you would remain online during a small and/or unsophisticated DDoS attack. However, if you are hit by a large attack, then any single-device solution will not be enough.
DDoS attack can make damage uniquely the destination IP or to the whole network that the DDoS packets go through?
Regardless of where the damage is seen, the solution often involves both finely targeted null-routing on the upstream ISP's network and local measures. Thus if your ISP really has no DDoS response capability, then you're at a disadvantage. But are you sure this is so -- have you asked them?
Just a side note, there are ISPs who specialize in providing connectivity with DDoS mitigation services built in. Examples might be Prolexic or Black Lotus; there are several others. If you feel at risk of being DDoS'ed, then using this kind of provider might be worth looking into.