It's like responding to a kid asking you why you said something.
S : "We need to get some milk."
R : "Why?"
S : "Because we need to bake a cake."
R : "Why?"
S : Because we have guests coming over tomorrow."
R : "Why?"
S "Because our uncle's making a stop at our city."
R : "Why?"
S : "He has a business conference at the next city over."
R : "Why?"
S : "His boss made him..."
R : "Why?"
S : "Ah, I don't know! You figure it out yourself."
R : "...Why?"
Surprisingly, the 5-Why's is a problem solving technique that requires thinking about the cause and effect of a situation. Sometimes done as a hypothesis, sometimes as evidence of a factual claim, it is effective in finding out the root cause of a given circumstance.