References and rhymes
Telling a designer, "I'll know it when I see it," is like asking a chef to cook a perfect steak without telling them how done they want it.
With enough patience and money, sure, you both could figure it out.
But designers can only perform guess work without some frame of reference. They need clear direction to do their best work.
As a client, you may rarely know exactly what you want, but if you can figure out what it should rhyme with, you can offer good direction.