Its a bit of gaming the system, but I see the logic in it.

Since self control is an important part of repaying debts, having a card (line of credit) that you could use, but don't, indicates you have a level of self control. There are people that, given any amount of credit line, will immediately run the debt up to the limit.

The measure of a person having a line of credit and not using it indicates to lenders you do have that level of self control. This isn't the only input used in a lending decision, but I can see the value of it when we're trying to determine one small test applied to general large populations.