It depends how clear your idea value is without it, or with a rough one, or mock ups vs polished.
There are a few goals you are trying to conquer with a prototype, overall its about concept clarity and valuation.
1. customer/investor acceptance: "i want that"
2. customer feedback: "you should change that"
3. investor valuation: "wow, you are that far along" vs "its just an idea"
Without more details only you know what it will take to get the right clarity to the customer and investors to answers on above.