[SkyMarshal]

>but isn't the point of borrowing against collateral that you have some utility from the collateral (e.g. a car you drive, a house you live in, part of a business you own) while you carry the loan?

Close, you can abstract "utility" one layer higher to "demand", or more accurately demand relative to supply. The thing you collateralize needs to be in demand by enough people to ensure it remains valuable over at least the life of the loan, but realistically much longer (so the lender is assured it will always be perceived by the market to have current and future value over any period during which the lender may need to reposses and resell the collateral). Utility gives things value which gives them demand, but it's really the demand that matters. There must always be a ready buyer for the collateral. Scarcity, like with Cryptokitties, gives things value too (rationally or not, but welcome to the human race).