In the case of being an investment property owner, consider a scenario where you own a few investment properties and that is your sole source of income. You would never have to work, have enough income to live by, and everything you do to those properties is a write off.
No, she doesn't get to write off her mortgage on the properties, but if the rent from the properties exceeds her mortgage (which is likely the case), and she has enough business deductions, she could live off that money alone - she would never have to have a 9-5 job. Eventually when she buys a new primary residence, the tax savings are incredible on that mortgage, which is being paid by your renters.