They're worth $80m today, and then maybe $82m next year, $84m the year after, and so on until they're worth $100m at maturity. (Obviously these numbers depend on current and future interest rates, and you'd be earning some interest in the meantime).

As I was trying to point out to the parent commenter, conflating "$100m today" with "$100m at maturity" leads to clear contradictions, like saying that a bank could earn $20m on paper simply by buying bonds trading below par value. Or to put it another way – if bank A holds $100m face value of 10-year bonds yielding 4%, and bank B holds $100m face value of 10-year bonds yielding 2% (but worth, say, $80m at market price), how can you claim that those banks are on equally good footing?

Valuing liquid bonds at par value is pretty clearly a hack to reduce volatility and increase confidence in banks' balance sheets, even if some people in the comments seem to view it as a more logical way of accounting. (Although to be clear, I don't mind companies doing their own fuzzy math as long as they give investors enough information to do proper due diligence. It's similar to the non-GAAP earnings that a lot of tech companies report.)