Banks like Westpac Banking Corp need debt and good margins to make their model profitable. In basic terms, a bank will take money from term deposit holders and 'wholesale debt investors' to lend that money out to homeowners, businesses and investors. The difference between what a bank pays to savings account and term deposit holders versus what it makes from mortgage holders (for example) is the net interest margin or NIM. When it comes to NIMs, the wider the margin, the better.