Banks need to have a certain amount of debt to make smart decisions about giving out loans. This is because they don't own the projects they finance, so they can't keep all the profits. Banks also check the risk of loans before giving them out. This explains why banks take on extra debt like deposits and loans, even if they already have enough money from deposits. It also explains why banks and finance companies have similar debt levels, even though finance companies don't take deposits like banks do.