Why do you need a discovery phase for a mobile app?

"I don't need that discovery phase, I know what I want.". 

"I know everything about my app, I can tell you those details."

That's the first reaction of many customers when we suggest them the discovery phase. Except for a rare few of them, nothing could be further from the truth. 

When a customer says that he has a clear picture of what they want, unless they have gone through multiple iterations of workflows, and wireframes, it's just a vague direction in which they want to move. 

Let me illustrate it with a simple example of  building a car. You come to us with a very clear specification of what you want, 4 wheels, roof, steering wheel, brakes, accelerator, leather seats, and you know for sure that you want it in black, car should be able to reach the speed of 100 miles per hour in very short time (you are not critical about it because you know you have a short budget). 

When I start to estimate this requirement, I get to following questions: 

1. Do you want a hatchback, a sedan, a convertible, a sports car or a truck?
2. Is it a diesel car or a petrol car? Or do you want CNG/LPG capabilities? Hybrid car with biofuel? Hybrid electric car?
3. What would be the engine capacity required? Any specification related to CC or torque?
4. What would be suspension type? 
5. How many airbags do you need? 2, 6 or no airbags ? Yeah, no airbags is also an option in certain markets.
6. Is it an automatic or manual gear shifting car?
7. You certainly don't want an electric car! (just making sure of it)
8. How many passengers, 2, 4, 5?
9. Do you want car entertainment system, Bluetooth, USB, audio only, video?
10. Do you want GPS navigation?
11. Do you want a rear view camera? 
12. Do you want height adjustable steering?
13. Do you want parking sensor?
14. Do you want fuel sensors indicating distance you can go till next refueling?
15. What kind of AC do you want? Standard or premium quality? (Depending on the weather conditions of target market you may want to decide that)
16. Do you want AC vents for the rear seats too?
17. Do you want Individual AC control?
18. Left-hand drive? or right-hand drive?
19. What is the length of the car? It can change the price of your car to the consumers. For example, there are more taxes on the cars of length above 4 meters in India.
20. What are your seat upholstery preferences? We understand that you want it to be leather seats, but do you want arm rests? For front as well as back seats? Do you want cup holder?
21. Do you want weather sensor for automatic rain wiper?
22. Do you want 2 doors or 4 doors?
23. Do you have any preference for ground clearance?
24. Do you have any preference for minimum turning radius?
25. Do you want fog lamps?
26. Do you want to offer a single model of the car or multiple models with varying features?
27. In the case of multiple models, how many, what features do you want in each model?
28. Which features can be given as add on?
29. Do you want extensive graphics on the car?
30. Do you want a sunroof?

And finally, 

Do you have any hard limit on the price of the car? 

Once you put that limit of car price, we have to reevaluate all the options in that light. We have to prioritize between options.

During discovery, we act as your consultant, we present you all these choices which you were unaware of. Even if you were aware of some of these, we would be providing insights into the repercussions of the snowball effect of each choice. 

Think of it, we just went through the options of a product which exist for more than a 100 years. You are bringing an innovative idea which has very few precedents and many of those precedents have been built at much higher costs. Doesn't it make sense to spend some time on getting the specifications in the detail before committing to a direction?