Imagine that you have to make a business case to your upper management for a product/project you want to get funded. There are two ways a product manager can ask for this:
1) I need $$$$$ and XXXXX number of people to do this project?
2) I have this idea that I have vetted with customers and prospects, here is the total size of the market, this will help us move the business forward, this would establish us as a market/thought leader, here is potential revenues we could bring in, what do you think and do you agree we need to do it?
Which do you think is going to be received well? Of course answer 2 (provided you have done enough research). The obvious question that will be asked would be – what would it take? And the answer is 1). But going in there with guns loaded just with 1) is not going to get anywhere.
The other benefit with 2), is you are asking for input whether it is the right thing to do – you are engaging your management to help you make the decision. Once you have the buy in that the idea/product is worth doing, they will open up for your justification for resources. But the common mistake made by product managers is doing 1) with no luck.