[jacquesm]

The second is wasteful, the first can be useful. It will help to plan before execution so you have a good mental model of the parameter space that your business is operating in which will help you with guidance when decisions need to be made. It would be delusional to think that you can completely model the problem before you have data so as you go forward you'll have to adjust the model and therefore the plan.

It basically is an instrument to cut down on the number of surprises that you'll encounter along the way and it gives you some measure of control.

Some people get stuck at the planning stage, they never make the jump to implementation, but at some point further planning becomes a pointless exercise and you just have to bite the bullet and do something.