Voting on the deal is necessary because this is likely to be the only sensible Withdrawal Agreement we can get. That actually means that there's a chance Parliament will accept it, even if it looks like a very slim chance. But even if not, then rejecting the deal allows Parliament to decide what to do next. Even "No Deal" covers a few different scenarios, and both sides will need time to prepare for these as far as possible.
The simple fact is that despite the denials of Brexiteers, No Deal is pretty terrible for all parties -- not least the UK. Neither side wants, then, to just sleepwalk into this. So one way or another Parliament must either avoid it or at least try to. It's blinkered to pretend that it is Parliament's responsibility to rip the UK out of the EU, and in doing so damage both sides economically and politically. It would be irresponsible and reckless to just let No Deal happen without exploring all other options.