Why Does A Project Manager Need To Understand Between Project And Product Life Cycle?

I thought that it is really essential to a manager knowing a project and its cycling product such that their customers could understand and feel enthusiastic on our product from our company.
This must be realized by them that all of their inquiries has been fulfilled if we as a manager is able to give a proper information to them in that they might feel contented on our products as well. So every kind of situation is getting conducive to them.
As a structural function holder in a company, a manager must have full knowledge either company mission, vision, procedure, programs or its product in order to transfer their knowledge to their down-liners below. So, a role model of a fabulous manager can be copied to.
Honestly, I feel that this question  is hard to answer specifically, but i have tried to do so.
Hopefully this can satisfy your intention.