Some prostitutes do pay their income tax, or at least a fair chunk of it. If you suddenly start buying houses and cars and you have no visible sign of income that tends to raise a red flag with CRA.
So smart prostitutes declare at least enough of their income to justify how they came up with the money for their house and their car and why their bank account shows a large positive balance and a healthy cash flow.
Same goes for anybody else running a cash business whether it's waitressing or running a laundrymat--declaring NOTHING is pretty stupid as it essentially locks you out of any normal sort of savings: No house, no car, no RRSP, no TFSA, no mutual funds, not even a savings account, is possible unless SOMETHING is declared.