There is a best practice that has been started a long time ago: somebody that has knowledge about project management must be engaged from the time where the idea came, no matter this person will be the assigned project manager to the project just in case the project is approved. If you review PRINCE2 you will find it inside the process life cycle.
In other cases that is the mix between business analysis and project management disciplines. You will find the recommendation I stated above just in case you review the IIBA or the PMI related standards.
Hope this helps!
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