as the immovable property appreciates, so do your assets and financing capacity. with a more valuable collateral asset - the bank will give a larger loan increasing real estate purchase prices push rent prices up as well. so may increased interest rates, since it's harder to buy
so by using real estate as collateral you're just tapping into the existing low interest liquidity/credit line at the same time the property earns a yield (e.g. via rents) and generally appreciates