Determine scenario for task or let user decide?

I've read material about user testing strategy and they all talk about determining a scenario for the task. Is this necessary?

Wouldn't it be more natural to let the user him/herself decide on the better, more realistic experience?

On the other hand, having a set scenario would mean results are more consistent. Have you any practice for this method? Do you see any problems with that?

Context: I am redesigning a website for renting rafts and I do not know if I should determine under what conditions (price, number of people, ...) should participants are ordering rafts