Why do borrowers pledge collateral?

"An impressive theoretical literature motivates collateral as a mechanism that reduces equilibrium credit rationing and other problems arising from asymmetric information between borrowers and lenders. However, no clear empirical evidence exists regarding the theory's central implication: that reducing asymmetric information reduces the incidence of collateral. We provide such evidence by exploiting exogenous variation in lender information sets related to their adoption of a new information technology and by comparing collateral outcomes before and after adoption. Our results are consistent with the central implication of the theoretical models and may also have efficiency and macroeconomic implications"--Federal Reserve Bank of Atlanta web site.