For one, the borrower does not accrue interest on any portion of the funds that are not being used. (reverse.mortgage)
If she does not use the line, she does not accrue interest on any funds she does not use and the line of credit grows on the unused portion. (reverse.mortgage)
Any over or under payment of accrued interest resulting from a rate change, will be applied to the borrower's balance. (wellsfargo.com)