Ultimately, it's about your story and what works for you, and it. The "What If" question is largely a starting point; of course other elements must end up in the story. Many of those are addressed in later assignments, but some things you'll have to come up with on your own. It's kind of like digging footers when building a house - the basic outline is laid out, but there's a lot more to the finished structure.

Science fiction puts a lot of emphasis on this particular question, but other genres use it as well. For example, in The Bourne Identity, it's "What if a master assassin woke up one day with amnesia?" Or, like Stephen King's Christine: "What if a car were possessed by evil spirits?"

(At least that's my interpretation. I didn't come up with the assignments.)