PMs will normally come up with the goals.
But designers will be better at coming up with solutions. They’re trained in a different way. And can think of all the different options, edge cases, etc.
If you’re a PM, don’t say: “We need to build a door knob. It’s should be this color and this material.”
It’s too specific. You’ll limit the designer’s creativity. You’ll be asking the designer into a person who just executes your designs.
Do say: “We need a mechanism to leave a room, then come back to the room later.”
Designers will tend to come up with better solutions, often things you would not have thought of.