How is the “why” relevant?
The 5th amendment protects him from having to answer that question.
The answer to the “why” question provides the background to the persons actions that night.
Possible scenarios (some good, some bad)
He is engaged in business where there are cash receipts such as a restaurant (tips and such)
He is engaged in a business that deals only on cash (or checks; many small businesses such as car repair do this) .
OR
another type of business that is doing a cash basis for nefarious reasons.
THAT is the defining question, and the reason for asking “why”?