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Why buy bonds in a no-arbitrage market?
For safety. If something catastrophic happens to your bank and your money is in there you will lose any not covered by FDIC. So if you have a very large amount of money you will store it in bonds as its much less likely that the US treasury will go bankrupt than your bank. I also literally just posted this in another thread: Certain rules and regulations penalize companies or institutions for holding cash, so they are shifting to bonds and bills. Fidelity, for example, is completely converting its $100 billion dollar cash fund to short term bills. Its estimated that over $2 trillion that is now in cash may be converted to bills, and that will obviously put upward preasure on the price of them. The treasury is trying to issue more short term debt to balance out the demand. read more here: http://www.wsj.com/articles/money-funds-clamor-for-short-term-treasurys-1445300813