In any business, clients are only willing to part with their money when they’re confident that there paying for something they need or want.
If your car breaks down, you pay a mechanic to fix it – they fulfil a need.
If you don’t particularly like the colour of your car, you can pay a painter to respray it for you – they fulfil a desire.
You don’t take your car in and say “Everything’s perfectly fine, I love the car. I just want you to spend a few days coming up with ideas and do some work on the car and if I like the changes, I’ll pay for your work”. That would be ridiculous.
>> Read: Nobody takes spec work seriously