Repo markets are systemically important funding markets, but are also used by firms to obtain the assets provided as collateral. Do these two functions complement each other? We build and estimate a model of repo trade between heterogeneous firms, and find that the answer is no: volumes and gains to trade would both be higher absent collateral demand. This is because on average the firms that need funding are also those that value the collateral to speculate or hedge interest rate risk. These results have implications for policies that affect collateral demand in repo markets, including rules on short selling.